V4L/DVB (12887): DIB7000P: SNR calcuation forr DiB7000P

Add the SNR monitoring for the dib7000p. The result is in dB.

Signed-off-by: Olivier Grenie <olivier.grenie@dibcom.fr>
Signed-off-by: Patrick Boettcher <patrick.boettcher@dibcom.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>