[media] dvb: make DVB frontend *_ops instances "const"

These are immutable.  Making them "const" allows the compiler to move
them to the "rodata" section.

Note that cxd2841er_t_c_ops cannot be made "const", because
cxd2841er_attach() modifies it.  Ouch!

[mchehab@s-opensource.com: fix merge conflicts]
Signed-off-by: Max Kellermann <max.kellermann@gmail.com>

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>