[media] rc-core: race condition during ir_raw_event_register()

A rc device can call ir_raw_event_handle() after rc_allocate_device(),
but before rc_register_device() has completed. This is racey because
rcdev->raw is set before rcdev->raw->thread has a valid value.

Cc: stable@kernel.org
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>