batman-adv: send GW_DEL event in case of soft-iface destruction

In case of soft_iface destruction send a GW DEL event to
userspace so that applications which are listening for GW
events are informed about the lost of connectivity and can
react accordingly.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>