V4L/DVB (6961): tda18271: move common code to tda18271-common.c

Move some common code to a new file to make this easier to look at.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>