ieee802154: 6lowpan: fix ARPHRD to ARPHRD_6LOWPAN

Currently there exists two interface types with ARPHRD_IEEE802154. These
are the 802.15.4 interfaces and 802.15.4 6LoWPAN interfaces. This is
more a bug because some userspace applications checks on this value like
wireshark. This occurs that wireshark will always try to parse a lowpan
interface as 802.15.4 frames. With ARPHRD_6LOWPAN wireshark will parse
it as IPv6 frames which is correct.

Much applications checks on this value to readout the EUI64 mac address
which should be the same for ARPHRD_6LOWPAN. BTLE 6LoWPAN and ieee802154
6LoWPAN will share now the same ARPHRD.

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