V4L/DVB (10804): Add support for ST STV0900 dual demodulator.

Add last piece of code to support ST STV0900 dual demodulator.
The IC consist of two dependent parts.
It may use single or dual mode.

Signed-off-by: Igor M. Liplianin <liplianin@netup.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>