dma: pl330: ensure DMA descriptors are zero-initialised

I see the following splat with 3.13-rc1 when attempting to perform DMA:

[  253.004516] Alignment trap: not handling instruction e1902f9f at [<c0204b40>]
[  253.004583] Unhandled fault: alignment exception (0x221) at 0xdfdfdfd7
[  253.004646] Internal error: : 221 [#1] PREEMPT SMP ARM
[  253.004691] Modules linked in: dmatest(+) [last unloaded: dmatest]
[  253.004798] CPU: 0 PID: 671 Comm: kthreadd Not tainted 3.13.0-rc1+ #2
[  253.004864] task: df9b0900 ti: df03e000 task.ti: df03e000
[  253.004937] PC is at dmaengine_unmap_put+0x14/0x34
[  253.005010] LR is at pl330_tasklet+0x3c8/0x550
[  253.005087] pc : [<c0204b44>]    lr : [<c0207478>]    psr: a00e0193
[  253.005087] sp : df03fe48  ip : 00000000  fp : df03bf18
[  253.005178] r10: bf00e108  r9 : 00000001  r8 : 00000000
[  253.005245] r7 : df837040  r6 : dfb41800  r5 : df837048  r4 : df837000
[  253.005316] r3 : dfdfdfcf  r2 : dfb41f80  r1 : df837048  r0 : dfdfdfd7
[  253.005384] Flags: NzCv  IRQs off  FIQs on  Mode SVC_32  ISA ARM  Segment kernel
[  253.005459] Control: 30c5387d  Table: 9fb9ba80  DAC: fffffffd
[  253.005520] Process kthreadd (pid: 671, stack limit = 0xdf03e248)

This is due to desc->txd.unmap containing garbage (uninitialised memory).

Rather than add another dummy initialisation to _init_desc, instead
ensure that the descriptors are zero-initialised during allocation and
remove the dummy, per-field initialisation.

Cc: Andriy Shevchenko <andriy.shevchenko@intel.com>
Acked-by: Jassi Brar <jassisinghbrar@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>