6LoWPAN-ND states [1] that the IPv6 address should be generated from the
EUI-64 of the interface, since it can be assumed as globally unique and
would not require duplicate address detection. Currently the xbee module is
not able to use any other address for short address with IPv6 since
NETCONF_OPT_SRC_LEN always returns 2. This patch fixes that.
[1] https://tools.ietf.org/html/rfc6775#section-5.2