batman-adv: split batadv_is_wifi_iface() into two functions

Previously batadv_is_wifi_iface() did two things at once: looking up a
net_device from an interface index, and determining if it is a wifi device.

The second part is useful itself when the caller already has a net_device
reference.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>