V4L/DVB (4940): Or51211: Changed SNR and signal strength calculations

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>