AA: ar71xx: fix MAC address assignment on the DIR-825-Bx boards

Backport of r35730.

Use the same MAC addresses as the original firmware.

Based on a patch from #10421.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Tested-by: Gabor Varga <vargagab@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/branches/attitude_adjustment@35731 3c298f89-4303-0410-b956-a3cf2f4a3e73