ar71xx: set proper wan mac during initial configuration on dir-825-c1

The goal is to set the WAN MAC address to the one on the
sticker on the bottom of the unit. Currently it is not
used at all. But some users expect the WAN interface to
have the MAC address that is written on the sticker.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38091 3c298f89-4303-0410-b956-a3cf2f4a3e73