keymap: Fix Touchpad Toggle on MSI Wind U90/U100

This key is handled by the hardware already, so handling it again in software
nullifies the effect. Newer kernels read the real state and send out a separate
KEY_TOUCHPAD_ON or KEY_TOUCHPAD_OFF event, so in both cases we need to ignore
that key.

https://bugs.freedesktop.org/show_bug.cgi?id=62404