[media] cx24120: Tidy up calls to dev_dbg

Remove __func__ from calls to dev_dbg as dynamic debug can add in
the function name anyway.
Remove debug call in dvbv3 read_signal_strength as userspace has
this value anyway.
Reword some strings to make them simpler / more obvious.

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>