ar71xx: local mac support on the wndr3700

The wndr3700 at least has no eth0 mac address and usually leverages
the first wireless device's mac when in a bridged scenario. If,
however, you want to route, and not bridge the interfaces, you
need a unique mac address for it.

This patch sets the local bit on the mac address pulled from the
wireless chip and uses the resulting address for eth0.

Patch-by: Dave Taht <dave.taht@gmail.com>

SVN-Revision: 29675