dmaengine: zxdma: fix memset call

Fix the call to memset in this driver
[linux-4.2-next-20150911/drivers/dma/zx296702_dma.c:444]: (warning)
memset() called to fill 0 bytes of 'ds'.

Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>