ath79: really fix TP-Link Archer C7 v2 MAC address

ath79: really fix TP-Link Archer C7 v2 MAC address

Revert 290c54473ead ("ath79: fix TP-Link Archer C7 v2 wlan1 MAC address")
which obviously aims to have a distinct MAC address per interface.

Unfortunally it doesn't match what is used by the stock firmware and we
shouldn'z use MAC Adresses not reserverd for/assigned to a particular
board.

The correct MAC adress increments for this board are:

wlan0 (5GHz) : -1
wlan1 (2.4GHz) : 0
eth1 (LAN) : 0
eth0 (WAN) : 1

Fixes: FS#408

Signed-off-by: Mathias Kresin <dev@kresin.me>