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>

drivers/dma/mv_xor.c

@@ -647,7 +647,7 @@ mv_xor_prep_dma_memcpy(struct dma_chan *chan, dma_addr_t dest, dma_addr_t src,
 
 	dev_dbg(mv_chan_to_devp(mv_chan),
 		"%s sw_desc %p async_tx %p\n",
-		__func__, sw_desc, sw_desc ? &sw_desc->async_tx : 0);
+		__func__, sw_desc, sw_desc ? &sw_desc->async_tx : NULL);
 
 	return sw_desc ? &sw_desc->async_tx : NULL;
 }