V4L/DVB (4028): Change dvb_frontend_ops to be a real field instead of a pointer field inside dvb_frontend

The dvb_frontend_ops is a pointer inside dvb_frontend. That's why every demod-driver
is having a field of dvb_frontend_ops in its private-state-struct and
using the reference for filling the pointer-field in dvb_frontend.
- It saves at least two lines of code per demod-driver,
- reduces object size (one less dereference per frontend_ops-access),
- be coherent with dvb_tuner_ops,
- makes it a little bit easier for newbies to understand how it works and
- avoids stupid mistakes because you would have to copy the dvb_frontend_ops
  always, before you could assign the static pointer directly, which was
  dangerous.
Signed-off-by: Patrick Boettcher <pb@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>