V4L/DVB (7575): mt312: add attach-time setting to invert lnb-voltage

Add a setting to config struct for inversion of lnb-voltage.
Needed for support of Avermedia A700 cards.

Signed-off-by: Matthias Schwarzott <zzam@gentoo.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>