[media] uvcvideo: small cleanup in uvc_video_clock_update()

Smatch is not smart enough to see that "&stream->clock.lock" and
"&clock->lock" are the same thing so it complains about the locking
here.  Let's make it more consistent.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>