ath79: fix MAC address setup for TP-Link TL-WDR3600/TL-WDR4300

The current ethernet MAC address setup of TL-WDR4300 board is different
from the setup of stock firmware:

OpenWrt: lan = label_mac -2, wan = label_mac -2
  stock: lan = label_mac,    wan = label_mac +1

The full address assignment is as follows:
LAN  label
WAN  label + 1
5G   label
2G   label - 1

This patch changes all devices using TL-WDR4300 board:
TL-WDR3600 v1 (checked on device)
TL-WDR4300 v1 (checked on device)
TL-WDR4300 v1 (IL)

Signed-off-by: Sungbo Eo <mans0n@gorani.run>
[rephrase/extend commit title/message, backport]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit a4260eaab7744c8e3f1f7a62a61aab5e3b562342)