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

This patch applies to all devices using TL-WDR4300 board:
TL-WDR3600 v1
TL-WDR4300 v1
TL-WDR4300 v1 (IL)
TL-WDR4310 v1
Mercury MW4530R v1

Signed-off-by: Sungbo Eo <mans0n@gorani.run>
(cherry picked from commit 9b02d32e)