lantiq/xrx200: fix WLAN button actions for Fritzbox 3370

lantiq/xrx200: fix WLAN button actions for Fritzbox 3370

The WLAN button actions are reversed, i.e. pressing the button emits a
'released' action, and vice versa.
This can easily be checked by adding
logger -t button_action "$BUTTON $ACTION"
as the second line of /etc/rc.button/rfkill, and using logread to read
the events (assuming the preceding patch has been applied).
Defining the GPIO as ACTIVE_LOW corrects this behavior.

Signed-off-by: Dustin Gathmann <dzsoftware@posteo.org>