media: si2168: Add spectrum inversion property

Some tuners produce inverted spectrum, but the si2168 is not
currently set up to accept it. This adds an optional parameter
to set the frontend up to receive inverted spectrum.

Parameter is optional and only boards who enable inversion
will utilize this.

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