media: rc: uevent sysfs file races with rc_unregister_device()

Only report uevent file contents if device still registered, else we
might read freed memory.

Reported-by:  <syzbot+ceef16277388d6f24898@syzkaller.appspotmail.com>
Cc: Hillf Danton <hdanton@sina.com>
Cc: <stable@vger.kernel.org> # 4.16+
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>