media: DaVinci-VPBE: constify vpbe_dev_ops

vpbe_dev_ops is only copied into the ops field at the end of a vpbe_device
structure, so it can be const.

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