[media] dvb-frontends/cxd2841er: support IF speed calc from tuner values

Add a AUTO_IFHZ flag and a function that will read IF speed values from any
attached tuner if the tuner supports this and if AUTO_IFHZ is enabled, and
else the passed default value (which probably matches Sony ASCOT tuners)
will be passed back. The returned value is then used to calculate the iffeq
which the demod will be programmed with.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Acked-by: Abylay Ospan <aospan@netup.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>