[media] s5p-mfc: Use NULL instead of 0 for pointer

Fixes the following warning:
drivers/media/platform/s5p-mfc/s5p_mfc_opr.c:56:27: warning:
Using plain integer as NULL pointer

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/platform/s5p-mfc/s5p_mfc_opr.c

@@ -53,7 +53,7 @@ void s5p_mfc_release_priv_buf(struct device *dev,
 {
 	if (b->virt) {
 		dma_free_coherent(dev, b->size, b->virt, b->dma);
-		b->virt = 0;
+		b->virt = NULL;
 		b->dma = 0;
 		b->size = 0;
 	}