media: uvcvideo: Constify video_subdev structures

uvc_subdev_ops is only passed as the second argument of
v4l2_subdev_init, which is const, so uvc_subdev_ops can be
const as well.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>