dmaengine: at_xdmac: lock fixes

Using _bh variant for spin locks causes this kind of warning:
Starting logging: ------------[ cut here ]------------
WARNING: CPU: 0 PID: 3 at /ssd_drive/linux/kernel/softirq.c:151
__local_bh_enable_ip+0xe8/0xf4()
Modules linked in:
CPU: 0 PID: 3 Comm: ksoftirqd/0 Not tainted 4.1.0-rc2+ #94
Hardware name: Atmel SAMA5
[<c0013c04>] (unwind_backtrace) from [<c00118a4>] (show_stack+0x10/0x14)
[<c00118a4>] (show_stack) from [<c001bbcc>]
(warn_slowpath_common+0x80/0xac)
[<c001bbcc>] (warn_slowpath_common) from [<c001bc14>]
(warn_slowpath_null+0x1c/0x24)
[<c001bc14>] (warn_slowpath_null) from [<c001e28c>]
(__local_bh_enable_ip+0xe8/0xf4)
[<c001e28c>] (__local_bh_enable_ip) from [<c01fdbd0>]
(at_xdmac_device_terminate_all+0xf4/0x100)
[<c01fdbd0>] (at_xdmac_device_terminate_all) from [<c02221a4>]
(atmel_complete_tx_dma+0x34/0xf4)
[<c02221a4>] (atmel_complete_tx_dma) from [<c01fe4ac>]
(at_xdmac_tasklet+0x14c/0x1ac)
[<c01fe4ac>] (at_xdmac_tasklet) from [<c001de58>]
(tasklet_action+0x68/0xb4)
[<c001de58>] (tasklet_action) from [<c001dfdc>]
(__do_softirq+0xfc/0x238)
[<c001dfdc>] (__do_softirq) from [<c001e140>] (run_ksoftirqd+0x28/0x34)
[<c001e140>] (run_ksoftirqd) from [<c0033a3c>]
(smpboot_thread_fn+0x138/0x18c)
[<c0033a3c>] (smpboot_thread_fn) from [<c0030e7c>] (kthread+0xdc/0xf0)
[<c0030e7c>] (kthread) from [<c000f480>] (ret_from_fork+0x14/0x34)
---[ end trace b57b14a99c1d8812 ]---

It comes from the fact that devices can called some code from the DMA
controller with irq disabled. _bh variant is not intended to be used in
this case since it can enable irqs. Switch to irqsave/irqrestore variant to
avoid this situation.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Cc: stable@vger.kernel.org # 4.0 and later
Signed-off-by: Vinod Koul <vinod.koul@intel.com>