[media] saa7146: make saa7146_use_ops const

Make these const as they are not modified in the file referencing them.
They are only used when their function pointer fields invokes a
function and therefore none of the structure fields are getting modified.
Also, add a const to the declaration in the header.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>