dmaengine: at_xdmac: Fix concurrency over chan's completed_cookie

dmaengine: at_xdmac: Fix concurrency over chan's completed_cookie

Caller of dma_cookie_complete is expected to hold a lock to prevent
concurrency over the channel's completed cookie marker. Call
dma_cookie_complete() with the lock held.

Fixes: e1f7c9eee707 ("dmaengine: at_xdmac: creation of the atmel eXtended DMA Controller driver")
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20211215110115.191749-5-tudor.ambarus@microchip.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>