fix: initialize gpio_keys input from pin state

Some kind of race condition caused the gpio_keys input module to
occasionally miss an event after boot. This is fixed by initializing the
pin state variable in the input_gpio_keys module.

Related to https://github.com/starcopter/bms-firmware/issues/6



Signed-off-by: Lasse Fröhner <lasse@starcopter.com>