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>

drivers/dma/sirf-dma.c

@@ -595,7 +595,7 @@ sirfsoc_dma_prep_cyclic(struct dma_chan *chan, dma_addr_t addr,
 	spin_unlock_irqrestore(&schan->lock, iflags);
 
 	if (!sdesc)
-		return 0;
+		return NULL;
 
 	/* Place descriptor in prepared list */
 	spin_lock_irqsave(&schan->lock, iflags);