batman-adv: deselect current GW on client mode switch off

When switching from gw_mode client to either off or server
the current selected gateway has to be deselected.
In this way when client mode is enabled again a gateway
re-election is forced and a GW_ADD event is consequently
sent.

The current behaviour instead is to keep the current gateway
leading to no GW_ADD event when gw_mode client is selected
for a second time

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