[media] cx23885-dvb: Remove a dirty hack that would require DVBv3

The cx23885-dvb driver has a dirty hack:
	1) it hooks the DVBv3 legacy call to FE_SET_FRONTEND;
	2) it uses internally the DVBv3 struct to decide some
	   configs.

Replace it by a change during the gate control. This will
likely work, but requires testing. Anyway, the current way
will break, as soon as we stop copying data for DVBv3 for
pure DVBv5 calls.

Compile-tested only.

Cc: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>