[media] DiB8000: improve the tuning and the SNR monitoring

This patches improves the tuning of the dib8000 and correct the monitoringr
of the SNR.The patch allows to faster detect that there is no ISDB-T channel
on the wanted frequency.

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>