ipv6: addrconf: drop ieee802154 specific things

This patch removes ARPHRD_IEEE802154 from addrconf handling. In the
earlier days of 802.15.4 6LoWPAN, the interface type was ARPHRD_IEEE802154
which introduced several issues, because 802.15.4 interfaces used the
same type.

Since commit 965e613d ("ieee802154: 6lowpan: fix ARPHRD to
ARPHRD_6LOWPAN") we use ARPHRD_6LOWPAN for 6LoWPAN interfaces. This
patch will remove ARPHRD_IEEE802154 which is currently deadcode, because
ARPHRD_IEEE802154 doesn't reach the minimum 1280 MTU of IPv6.

Also we use 6LoWPAN EUI64 specific defines instead using link-layer
constanst from 802.15.4 link-layer header.

Cc: David S. Miller <davem@davemloft.net>
Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Cc: James Morris <jmorris@namei.org>
Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Cc: Patrick McHardy <kaber@trash.net>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>