media: dvb-frontends/stv0367: split SNR determination into functions

The read_snr() functions currently do some magic to return relative scale
values when called. Split out register readouts into separate functions
so the functionality can be reused in some other way.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>