batman-adv: Increase hard_iface refcnt for ptype

The hard_iface is referenced in the packet_type for batman-adv. Increase
the refcounter of the hard_interface for it to have an explicit reference
for it in case this functionality gets refactorted and the currently
used implicit reference for it will be removed.

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