[media] rc: Pospone ir raw decoders loading until really needed

This changes rc_core to not load the IR decoders at load time,
postponing it to load only if a RC_DRIVER_IR_RAW device is
registered via rc_register_device.
We use a static boolean variable, to ensure decoders modules
are only loaded once.
Tested with rc-loopback device only.

Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
Acked-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>