ath79: fix MAC address for Buffalo BHR-4GRV
I added mtd-mac-address for WZR-HP-G450H and BHR-4GRV in
1df1ea4d7e8b6ae3351780ed58800ccd9edd4c27, but that address in ART is
incorrect for BHR-4GRV.
WZR-HP-G450H has wlan eeprom and MAC address in ART, but BHR-4GRV
has only MAC address in ART.
- WZR-HP-G450H
  - eeprom: 0x1000
  - MAC:    0x1002
- BHR-4GRV
  - MAC:    0x0
Signed-off-by: INAGAKI Hiroshi <[email protected]>