rt2x00: Fix sparse warning on nested container_of()

Sparse produces warnings about nested contain_of() statements,
this means that lines like:
	interface_to_usbdev(to_usb_interface(rt2x00dev->dev));
will upset sparse.
Add a new macro to rt2x00usb.h which will convert to device
structure to the usb_device pointer in 2 steps to prevent this
sparse warning.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2x00usb.h

@@ -26,6 +26,12 @@
 #ifndef RT2X00USB_H
 #define RT2X00USB_H
 
+#define to_usb_device_intf(d) \
+({ \
+	struct usb_interface *intf = to_usb_interface(d); \
+	interface_to_usbdev(intf); \
+})
+
 /*
  * This variable should be used with the
  * usb_driver structure initialization.