V4L/DVB (4939): Or51132: Changed SNR and signal strength reporting

Removes embedded log functions and makes use of the DVB math functions
to provide SNR in dB.  The changes are modeled after recent changes made
to the LGDT330x frontends in lgdt330x.c

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