batman-adv: Move deinitialization of soft-interface to destructor

The deinitialization of the soft-interface created in ndo_init/constructor
should be done in the destructor and not directly before calling
unregister_netdevice

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Acked-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>