sunxi: Ensure that the NIC specific bytes of the mac are not all 0

sunxi: Ensure that the NIC specific bytes of the mac are not all 0

On 2 of my H3 boards bytes 13-15 of the SID are all 0 leading to
the NIC specific bytes of the mac all being 0, which leads to the
boards not getting an ipv6 address from the dhcp server.

This commits adds a check to ensure this does not happen.

Cc: Chen-Yu Tsai <wens@csie.org>
Cc: Corentin LABBE <clabbe.montjoie@gmail.com>
Cc: Amit Singh Tomar <amittomer25@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>