media: mxl5xx: constify dvb_frontend_ops structure

The dvb_frontend_ops structure is only copied into the ops field
of a dvb_frontend structure, so it can be const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>