media: platform: sunxi: sun6i-csi: fix error return code of sun6i_video_start_streaming()

When sun6i_video_remote_subdev() returns NULL to subdev, no error return
code of sun6i_video_start_streaming() is assigned.
To fix this bug, ret is assigned with -EINVAL in this case.

Reported-by: TOTE Robot <[e-mail]>
Signed-off-by: Jia-Ju Bai <[e-mail]>
Fixes: 5cc7522d8965 ("media: sun6i: Add support for Allwinner CSI V3s")
Acked-by: Chen-Yu Tsai <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/platform/sunxi/sun6i-csi/sun6i_video.c

···
 	}
 
 	subdev = sun6i_video_remote_subdev(video, NULL);
-	if (!subdev)
+	if (!subdev) {
+		ret = -EINVAL;
 		goto stop_media_pipeline;
+	}
 
 	config.pixelformat = video->fmt.fmt.pix.pixelformat;
 	config.code = video->mbus_code;