Staging: batman-adv: Don't call free_netdev twice

Free_netdev is registered as destructor in interface_setup for every
soft_device. This destructor is automatically called from
unregister_netdev and we must not call it again for the freed
net_device.

Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>