[media] uvcvideo: Lock controls mutex when querying menus

uvc_find_control() must be called with the controls mutex locked. Fix
uvc_query_v4l2_menu() accordingly.

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