V4L/DVB (7656): tda826x: Calculate cut off fequency from symbol rate

This patch makes the tuner work with transonders providing higher
symbol rates.
It was contributed by Oliver Endriss.

Signed-off-by: Hartmut Hackmann <hartmut.hackmann@t-online.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>