rt2x00: Endless loop on hub port power down

I've met an endless (or at least very long) loop if I power down the usb
port on witch a usb wifi key is plugged.
(Ok, it's not very smart to power down a usb port when a usb key is in
used... but still, I think that should not lead to an endless loop).

I have a lot of:
ieee80211 phy1: rt2x00usb_vendor_request: Error - Vendor Request 0x07 failed for offset 0x0438 with error -71

(-71==-EPROTO)

How to reproduce:
- plug an usb wifi key
- ip link set wlan0 up
- hub-ctrl -b usb_bus -d usb_device -P usb_port -p 0

hub-ctrl source: https://github.com/codazoda/hub-ctrl.c/blob/master/hub-ctrl.c



The following patch prevents the endless loop, but I'm really not sure
that The Right Way To Do It (R)

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>