[media] cx24120: Remove additional calls to read_status

get_frontend was making a call to read_status, which is being
called periodically anyway. Have read_status store fe_status in
the state struct instead and use that within get_frontend.

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>