[media] rc-core: allow calling rc_open with device not initialized

The device initialization completes only after calling
input_register_device(). However, rc_open() can be called while
the device is being registered by the input/evdev core. So, we
can't expect that rc_dev->initialized to be true.

Change the logic to don't require initialized == true at rc_open
and change the type of initialized to be atomic.

this way, we can check for it earlier where it is really needed,
without needing to lock the mutex just for testing it.

Tested with nuvoton_cir driver on a NUC5i7RYB with CIR integrated on it.

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