[media] media: mem2mem: make reference to struct m2m_ops in the core const

The mem2mem core doesn't change struct m2m_ops, provided by the driver,
make references to it const.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>