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>
Cc: Mauro Carvalho Chehab <mchehab@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>