[media] cx24120: Convert read_ber to retrieve from cache

Instead of reading BER again for DVBv3 call, use the value from the cache.

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>