V4L/DVB (3795): Fix for CX24123 & low symbol rates

- fixed the reception of channels with low symbol rates.
  ( The VGA1 and VGA2 offsets recommended by cx24109 docs for
  symbol rates from 1 to 5 MSps do not work. I changed them
  to values found experimentally. The charge pump current
  and FILTUNE voltage are now set to values recommended in
  the docs. This improves reception for symbol rates < 15 MSps.
  The values written in the SYSSymbolRate registers are calculated
  with better precision. )

- fixed the cx24123_get_fec() function. It was returning the values
  for DCII mode.
- removed some unused variables

Signed-off-by: Vadim Catana <skystar at moldova.cc>
Signed-off-by: Andrew de Quincey <adq_dvb@lidskialf.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>