ieee802154: fix iface dump with lowpan

This patch adds a hacked solution for an interface dump with a running
lowpan interface. This will crash because lowpan and wpan interface use
the same arphdr. To change the arphdr will change the UAPI, this patch
checks on mtu which should on lowpan interface always different than
IEEE802154_MTU.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>