Input: atkbd - throttle LED switching

On some boxes keyboard controllers are too slow to withstand
continuous flow of requests to turn keyboard LEDs on and off
and start losing some keypresses or even all of them.

Delay executing of LED switching request if we had another one
within 50 ms thus easing load on the controller.

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