V4L/DVB (4012): Fix cx24123 diseqc

Rework diseqc support to be more in line with the other demod drivers.
Fix Nova-S-Plus/Nova-SE2 diseqc.
Cleanup API.

Signed-off-by: Andrew de Quincey <adq_dvb@lidskialf.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>