batman-adv: invoke dev_get_by_index() outside of is_wifi_iface()

Upcoming changes need to perform other checks on the
incoming net_device struct.

To avoid performing dev_get_by_index() for each and every
check, it is better to move it outside of is_wifi_iface()
and search the netdev object once only.

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