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>