V4L/DVB (3804): Tweak bandselect setup fox cx24123

*) Allow forcing the bandselect value with a module parameter to
facilitate determining the correct bandselect frequencies.
*) Changes the bandselect frequency thresholds based on experiments
with the above parameter in conjunction with the values in the spec.

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