ipv6: addrconf: fix 48 bit 6lowpan autoconfiguration

This patch adds support for 48 bit 6LoWPAN address length
autoconfiguration which is the case for BTLE 6LoWPAN.

Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>