good idea to discard any character other than ACK/NAK during probe
       it causes missing releases and keys getting "stuck" when a command
       issued on enabled device. The effect is easily demonstrated with
       the following command:
           while true; do xset led 3; xset -led 3; done

       With this change extra characters will be discarded only if device
       has not been marked as "enabled" yet.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>