dmaengine: mediatek: mtk-hsdma: use NULL instead of using plain integer as pointer

This fixes the following sparse warnings:
drivers/dma/mediatek/mtk-hsdma.c:604:26: warning: Using plain integer
as NULL pointer
Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Link: https://lore.kernel.org/r/1649750340-30777-1-git-send-email-baihaowen@meizu.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/mediatek/mtk-hsdma.c

@@ -601,7 +601,7 @@ static void mtk_hsdma_free_rooms_in_ring(struct mtk_hsdma_device *hsdma)
 			cb->flag = 0;
 		}
 
-		cb->vd = 0;
+		cb->vd = NULL;
 
 		/*
 		 * Recycle the RXD with the helper WRITE_ONCE that can ensure