[media] cx231xx: Improve signal reception for PV SBTVD

Instead of using 3.3 MHz IF, use 4MHz. That's the standard
value for the demod, and, while it can be adjusted, 3.3 MHz
is out of the recommended range. So, let's stick with the
default.
With regards to the IF voltage level, instead of using
0.5 V(p-p) for IF, use 2V, giving a 12dB gain.
The rationale is that, on PixelView SBTVD Hybrid,
even 2V(p-p) would be in the nominal range for IF,
as the maximum range on this particular device is 3V.
A higher gain here should help to improve reception under
weak signals.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>