ar71xx: fix mac address calculation on the OpenMesh MR600

Signed-off-by: Marek Lindner <marek@open-mesh.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35446 3c298f89-4303-0410-b956-a3cf2f4a3e73