ath79: remove wmac mtd-mac-address for UniFi AC family

ath79: remove wmac mtd-mac-address for UniFi AC family

The MAC address for the wmac 2.4 GHz radio of the Ubiquiti UniFi AC
family of devices is actually embedded in the mtd-cal-data, so there
is no need for mtd-mac-address (which was incorrectly forcing wmac
to have the same MAC as eth0). This makes it coherent with the stock
firmware and the ar71xx target:

 · XX:XX:XX:X0:XX:XX eth0
 · XX:XX:XX:X1:XX:XX ath0/wlan1 (2.4 GHz)
 · XX:XX:XX:X2:XX:XX ath1/wlan0 (5 GHz)

Checked on a UniFi AC Mesh, a UniFi AC LR and a UniFi Lite.

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>
(cherry picked from commit 20ace70db65c3f1cb6a842d3092ac2eb7be81b5a)