V4L/DVB (4789): Lgdt330x: SNR and signal strength reporting

Update the SNR calculations to use the new dvb_math log function, and add
SNR calculations for all supported modulations for both lg dt3302 and dt3303.
The QAM equations don't appear in the dt3302 datasheet, so the ones from the
dt3303 datasheet were used.
SNR returned is the actual value in dB as 8.8 fixed point.
Reporting of real signal strength isn't supported, so rather than return 0,
which confuses some software and users, a re-scaled SNR value is returned.
Code originally by Rusty Scott.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Rusty Scott <rustys@ieee.org>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>