dma: mv_xor: use NULL instead of 0

%p is used, thus NULL should be used instead of 0
in order to fix the following sparse warning:

drivers/dma/mv_xor.c:648:9: warning: Using plain integer as NULL pointer

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>