dmaengine: pl330: Fix NULL pointer dereference on driver unbind

Fix a NULL pointer dereference after unbinding the driver, if channel
resources were not yet allocated (no call to
pl330_alloc_chan_resources()):
$ echo 12850000.mdma > /sys/bus/amba/drivers/dma-pl330/unbind
[   13.606533] DMA pl330_control: removing pch: eeab6800, chan: eeab6814, thread:   (null)
[   13.614472] Unable to handle kernel NULL pointer dereference at virtual address 0000000c
[   13.622537] pgd = ee284000
[   13.625228] [0000000c] *pgd=6e1e4831, *pte=00000000, *ppte=00000000
[   13.631482] Internal error: Oops: 17 [#1] PREEMPT SMP ARM
[   13.636859] Modules linked in:
[   13.639903] CPU: 0 PID: 1 Comm: sh Not tainted 3.17.0-rc3-next-20140904-00004-g7020ffc33ca3-dirty #420
[   13.649187] task: ee80a800 ti: ee888000 task.ti: ee888000
[   13.654589] PC is at _stop+0x8/0x2c8
[   13.658131] LR is at pl330_control+0x70/0x2e8
[   13.662468] pc : [<c0206028>]    lr : [<c020649c>]    psr: 60000093
[   13.662468] sp : ee889e58  ip : 00000001  fp : 000bab70
[   13.673922] r10: eeab6814  r9 : ee16debc  r8 : 00000000
[   13.679131] r7 : eeab685c  r6 : 60000013  r5 : ee16de10  r4 : eeab6800
[   13.685641] r3 : 00000002  r2 : 00000000  r1 : 00010000  r0 : 00000000
[   13.692153] Flags: nZCv  IRQs off  FIQs on  Mode SVC_32  ISA ARM  Segment user
[   13.699357] Control: 10c5387d  Table: 6e28404a  DAC: 00000015
[   13.705085] Process sh (pid: 1, stack limit = 0xee888240)
[   13.710466] Stack: (0xee889e58 to 0xee88a000)
[   13.714808] 9e40:                                                       00000002 eeab6800
[   13.722969] 9e60: ee16de10 eeab6800 ee16de10 60000013 eeab685c c020649c 00000000 c040280c
[   13.731128] 9e80: ee889e80 ee889e80 ee16de18 ee16de10 eeab6880 eeab6814 00200200 eeab68a8
[   13.739287] 9ea0: 00100100 c0208048 00000000 c0409fc4 eea80800 eea808f8 c0605c44 0000000e
[   13.747446] 9ec0: 0000000e eeb3960c eeb39600 c0203c48 eea80800 c0605c44 c0605a8c c023f694
[   13.755605] 9ee0: ee80a800 eea80834 eea80800 c023f704 ee80a800 eea80800 c0605c44 c023e8ec
[   13.763764] 9f00: 0000000e ee149780 ee29e580 ee889f80 ee29e580 c023e19c 0000000e c01167e4
[   13.771923] 9f20: c01167a0 00000000 00000000 c0115e88 00000000 00000000 ee0b1a00 0000000e
[   13.780082] 9f40: b6f48000 ee889f80 0000000e ee888000 b6f48000 c00bfadc 00000000 00000003
[   13.788241] 9f60: 00000000 00000000 00000000 ee0b1a00 ee0b1a00 0000000e b6f48000 c00bfdf4
[   13.796401] 9f80: 00000000 00000000 ffffffff 0000000e b6f48000 b6edc5d0 00000004 c000e7a4
[   13.804560] 9fa0: 00000000 c000e620 0000000e b6f48000 00000001 b6f48000 0000000e 00000000
[   13.812719] 9fc0: 0000000e b6f48000 b6edc5d0 00000004 0000000e b6f4c8c0 000c3470 000bab70
[   13.820879] 9fe0: 00000000 bed2aa50 b6e18bdc b6e6b52c 60000010 00000001 c0c0c0c0 c0c0c0c0
[   13.829058] [<c0206028>] (_stop) from [<c020649c>] (pl330_control+0x70/0x2e8)
[   13.836165] [<c020649c>] (pl330_control) from [<c0208048>] (pl330_remove+0xb0/0xdc)
[   13.843800] [<c0208048>] (pl330_remove) from [<c0203c48>] (amba_remove+0x24/0xc0)
[   13.851272] [<c0203c48>] (amba_remove) from [<c023f694>] (__device_release_driver+0x70/0xc4)
[   13.859685] [<c023f694>] (__device_release_driver) from [<c023f704>] (device_release_driver+0x1c/0x28)
[   13.868971] [<c023f704>] (device_release_driver) from [<c023e8ec>] (unbind_store+0x58/0x90)
[   13.877303] [<c023e8ec>] (unbind_store) from [<c023e19c>] (drv_attr_store+0x20/0x2c)
[   13.885036] [<c023e19c>] (drv_attr_store) from [<c01167e4>] (sysfs_kf_write+0x44/0x48)
[   13.892928] [<c01167e4>] (sysfs_kf_write) from [<c0115e88>] (kernfs_fop_write+0xc0/0x17c)
[   13.901090] [<c0115e88>] (kernfs_fop_write) from [<c00bfadc>] (vfs_write+0xa0/0x1a8)
[   13.908812] [<c00bfadc>] (vfs_write) from [<c00bfdf4>] (SyS_write+0x40/0x8c)
[   13.915850] [<c00bfdf4>] (SyS_write) from [<c000e620>] (ret_fast_syscall+0x0/0x30)
[   13.923392] Code: e5813010 e12fff1e e92d40f0 e24dd00c (e590200c)
[   13.929467] ---[ end trace 10064e15a5929cf8 ]---

Terminate the thread and free channel resource only if channel resources
were allocated (thread is not NULL).

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: <stable@vger.kernel.org>
Fixes: b3040e40 ("DMA: PL330: Add dma api driver")
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>