batman-adv: remove broadcast packets scheduled for purged outgoing if

When an interface is purged, the broadcast packets scheduled for this
interface should get purged as well.

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