batman-adv: send GW_DEL event when the gw client mode is deselected

Whenever the GW client mode is deselected, a DEL event has
to be sent in order to tell userspace that the current
gateway has been lost. Send the uevent on state change only
if a gateway was currently selected.

Reported-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>