AmigaOneG3SE/enet: get mac address from environment

Always use the MAC address that is stored in the environment first before
falling back to the ROM.  This also cuts out any comparison steps: if the
mac in the env is sane, the ROM is never consulted.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
CC: Thomas Frieden <ThomasF@hyperion-entertainment.com>
CC: Ben Warren <biggerbadderben@gmail.com>