[media] cx24120: Move CNR to DVBv5 stats

Better to report CNR via DVBv5 stats, as scale can be used.

Signed-off-by: Jemma Denson <jdenson@gmail.com>
Signed-off-by: Patrick Boettcher <patrick.boettcher@posteo.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>