[media] cx24120: Check for lock before updating BER & UCB

BER & UCB aren't available unless we're locked; don't update dvbv5
stats when not locked and mark these counters as unavailable.

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>