lantiq/xrx200: make WLAN button responsive on Fritzbox 7360 & 7362

lantiq/xrx200: make WLAN button responsive on Fritzbox 7360 & 7362

Pressing the 'WLAN' button should enable/disable wireless activity.
Currently, the button is mapped to the KEY_WLAN, which will not
have this effect.
This patch changes the mapping of the WLAN button, so a button
press will emit an action for the 'rfkill' key instead of 'wlan'.
Apparently, this is what stock OpenWRT expects.

This fix is analogous to the preceding patch for Fritzbox 3370.

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