ramips: Fix Hongdian H7920 v40 mac address
After extracting the EEPROMs of different devices, only the 0x4 address is unique.
Use the 0x4 address as the LAN address, and the LAN+1 address as the WAN address.
Signed-off-by: Coia Prant <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20256
Signed-off-by: Hauke Mehrtens <[email protected]>