Add support for multiple AP configurations to rt2x00 wifi devices

rt2x00/ralink devices are very common now - most of the cheap USB wifi dongles on ebay seem to have this chipset. They should support multiple access points on a single device - i.e. run two wireless networks like a private and a guest network from the one device. 

The existing code doesn't cope with this and if you try to set up a second network (wlan0_0) it just tells you the device is already busy. I found a patch for this problem on openwrt which applied cleanly to the source (http://patchwork.openwrt.org/patch/2928/) and works for me.