[media] cx23885-dvb: use a better approach to hook set_frontend

When the frontend drivers got converted to DVBv5 API, the original
hook that tracked when a frontend is set got removed, being replaced
by an approach that would use the gate control. That doesn't work
fine with some boards. Also, the code were called more times than
desired.
Replace it by a logic that will hook the dvb set_frontend ops,
with works with both DVBv3 and DVBv5 calls.
Tested on a Mygica X8502 OEM board.

Tested-by: Alfredo Delaiti <alfredodelaiti@netscape.net>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>