rt2x00: add device id for windy31 usb device

This patch adds the device id for the windy31 USB device to the rt73usb
driver.

Thanks to Ralf Flaxa for reporting this and providing testing and a
sample device.

Reported-by: Ralf Flaxa <rf@suse.de>
Tested-by: Ralf Flaxa <rf@suse.de>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>