batman-adv: Drop reference to netdevice on last reference

The references to the network device should be dropped inside the release
function for batadv_hard_iface similar to what is done with the batman-adv
internal datastructures.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>