media: rc: ensure that uevent can be read directly after rc device register

commit 896111dc upstream.

There is a race condition where if the /sys/class/rc0/uevent file is read
before rc_dev->registered is set to true, -ENODEV will be returned.

Link: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1901089



Cc: stable@vger.kernel.org
Fixes: a2e2d73f ("media: rc: do not access device via sysfs after rc_unregister_device()")
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>