[media] s5p-tv: fix error return code in mxr_acquire_video()

Fix to return a negative error code in the vb2_dma_contig_init_ctx()
error handling case instead of 0, as done elsewhere in this function.
Also vb2_dma_contig_init_ctx() return ERR_PTR() in case of error and
never return NULL, so use IS_ERR() replace IS_ERR_OR_NULL().
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/platform/s5p-tv/mixer_video.c

@@ -81,8 +81,9 @@ int mxr_acquire_video(struct mxr_device *mdev,
 	}
 
 	mdev->alloc_ctx = vb2_dma_contig_init_ctx(mdev->dev);
-	if (IS_ERR_OR_NULL(mdev->alloc_ctx)) {
+	if (IS_ERR(mdev->alloc_ctx)) {
 		mxr_err(mdev, "could not acquire vb2 allocator\n");
+		ret = PTR_ERR(mdev->alloc_ctx);
 		goto fail_v4l2_dev;
 	}