[media] media: rc: remove unneeded mutex in rc_register_device

Access to dev->initialized is atomic and dev->initialized isn't
accessed in any other code protected by this mutex.
Therefore we don't need to get the mutex here.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>