[media] v4l2-ctrls: fix missing 'read-only' check

VIDIOC_S_CTRL did not check against read-only controls. Even worse, for
controls of type CTRL_CLASS it would cause a kernel oops since those controls
do not have a s_ctrl op.
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/video/v4l2-ctrls.c

@@ -1824,6 +1824,9 @@ static int set_ctrl(struct v4l2_ctrl *ctrl, s32 *val)
 	int ret;
 	int i;
 
+	if (ctrl->flags & V4L2_CTRL_FLAG_READ_ONLY)
+		return -EACCES;
+
 	v4l2_ctrl_lock(ctrl);
 
 	/* Reset the 'is_new' flags of the cluster */