V4L/DVB (3803): Various correctness fixes to tuning.

*) Sets an additional tuner parameter (demodulator sample gain) that
wasn't being set before.
*) Removes the low symbol rate tuner parameter tweaks in the previous
patch -- it appears those tweaks are not necessary with the demodulator
sample gain set correctly.
*) Cleanup and document the demodulator register initialization sequence.
*) Change set_fec routine to disable FEC auto scan when a specific code
rate is selected.
*) Remove error message when reported FEC is invalid (which happens
sometimes when the card has no signal)

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>