[media] uvcvideo: Fix uvc_query_v4l2_ctrl() and uvc_xu_ctrl_query() locking

Take the ctrl_mutex mutex before touching control information in those
functions.

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