V4L/DVB (13377): make struct videobuf_queue_ops constant

The videobuf_queue_ops function vector is not declared constant, but
there's no need for the videobuf layer to ever change it.  Make it const
so that videobuf users can make their operations const without warnings.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>