ipq806x: ubnt,unifi-ac-hd: reorder eth0 and eth1

ipq806x: ubnt,unifi-ac-hd: reorder eth0 and eth1

The Ubiquiti UniFi AC HD (UAP-AC-HD, UAP301) has two Ethernet ports,
labeled MAIN and SECONDARY, connected to gmac2 and gmac1, respectively.
The standard probe order results in gmac1/SECONDARY being eth0 and
gmac2/MAIN being eth1. This does not match the stock firmware, is
contrary to user expectation, causes the wrong (high) MAC address to be
used in a bridged configuration (the default for this device), and makes
the gmac2/MAIN port unusable in the preinit environment (such as for
failsafe). Until a recent patch, gmac1/SECONDARY (eth0) was not even
usable.

This reorders the ports so that gmac2/MAIN is eth0, and the now-working
gmac1/SECONDARY is eth1. eth0 has the low MAC address and eth1 has the
high; when bridged, the bridge takes on the correct low MAC address.
This matches the stock firmware. The MAIN port is usable for failsafe
during preinit.

This device does not have a switch on board, so there's no possibility
to remap ports via switch configuration. "ip link set $interface name"
is used instead, during preinit before networking is configured.

Signed-off-by: Mark Mentovai <mark@moxienet.com>
Build-tested: ipq806x/ubnt,unifi-ac-hd
Run-tested: ipq806x/ubnt,unifi-ac-hd