[media] tw5864: constify vb2_ops structure

Inspired by "[media] pci: constify vb2_ops structures" patch
from Julia Lawall (dated 9 Sep 2016).

Signed-off-by: Andrey Utkin <andrey.utkin@corp.bluecherry.net>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>