media: ngene: deduplicate I2C adapter evaluation

The I2C adapter evaluation (based on chan->number) is duplicated at
several places (tuner_attach_() functions, demod_attach_stv0900() and
cineS2_probe()). Clean this up by wrapping that construct in a separate
function which all users of that can pass the ngene_channel pointer and
get the correct I2C adapter from.

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