Convert MAC address and label_mac configuration of Buffalo
WSR-1166DHP to use the generic function of OpenWrt.
Apply commit 770cfe9 for WCR-1166DS to WSR-1166DHP too.
Tested on the device and MAC address is kept before and after this
change.
Signed-off-by: Kazuhiro Ito <kzhr@d1.dion.ne.jp>