[media] cx24120: Return DVBv3 signal strength from cache

This patch changes DVBv3 signal strength to read from the cache by
moving the signal strength reading routine into get_stat, and
reducing read_signal_strength to just returning the cached value.

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>