media: rc: mce_kbd decoder: fix race condition

The MCE keyboard sends both key down and key up events. We have a timeout
handler mce_kbd_rx_timeout() in case the keyup event is never received;
however, this may race with new key down events from occurring.

The race is that key down scancode arrives and key down events are
generated. The timeout handler races this and generates key up events
straight afterwards. Since the keyboard generates scancodes every 100ms,
most likely the keys will be repeated 100ms later, and now we have new
key down events and the user sees duplicate key presses.

Reported-by: Matthias Reichl <hias@horus.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>