[media] cx24120: Tidy cx24120_init

Use reg instead of ret for all calls to readreg, remove ret_EA
as it doesn't need to be a separate var anymore.

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>