[media] cx24120: make sure tuner is locked at get_frontend

Change get_frontend to re-check current lock status rather than relying
on a cached value from get_status. Removes potential for tuning failure
if get_frontend is called during tuning.

Probably not too essential as other changes work around this:
https://patchwork.linuxtv.org/patch/32845/



Signed-off-by: Jemma Denson <jdenson@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>