media: cx231xx: Add second i2c demod to Hauppauge 975

Hauppauge HVR-975 is a hybrid, dual frontend, single tuner USB device.
It contains lgdt3306a and si2168 frontends and one si2157 tuner. The
lgdt3306a frontend is currently enabled. This creates the second
demodulator and attaches it to the tuner.

Enables lgdt3306a|si2168 + si2157

Signed-off-by: Brad Love <brad@nextdimension.cc>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>