media: platform: constify videobuf_queue_ops structures

videobuf_queue_ops are not supposed to change at runtime. All functions
working with videobuf_queue_ops provided by <media/videobuf-core.h> work
with const videobuf_queue_ops. So mark the non-const structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>