dma: sirf: use NULL instead of 0

sirfsoc_dma_prep_cyclic() returns pointer, thus NULL should be
used instead of 0 in order to fix the following sparse warning:

drivers/dma/sirf-dma.c:598:24: 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>