V4L/DVB: uvcvideo: Check uvc_ctrl_begin return value in VIDIOC_S_CTRL

The function return value wasn't checked due to a missing variable
assignment.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/uvc/uvc_v4l2.c

@@ -539,7 +539,7 @@ static long uvc_v4l2_do_ioctl(struct file *file, unsigned int cmd, void *arg)
 		xctrl.id = ctrl->id;
 		xctrl.value = ctrl->value;
 
-		uvc_ctrl_begin(chain);
+		ret = uvc_ctrl_begin(chain);
 		if (ret < 0)
 			return ret;