media: dvb-frontends/stv0367: DVB-C signal strength statistics

Provide QAM/DVB-C signal strength in decibel scale. Values returned from
stv0367cab_get_rf_lvl() are good but need to be multiplied as they're in
1dBm precision.

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>