media: dvb-frontends/stv0910: Add demod-only signal strength reporting

Original code at least has some signed/unsigned issues, resulting in
values like 32dBm. Implement signal strength readout to work without
asking the attached tuner, and use a lookup table instead of log calc.
Values reported appear plausible, gathered from feedback from several
testers.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Tested-by: Richard Scobie <r.scobie@clear.net.nz>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>