usb_sg_cancel — 
     stop scatter/gather i/o issued by usb_sg_wait
 
| void usb_sg_cancel ( | struct usb_sg_request * io ); | 
   This stops a request after it has been started by usb_sg_wait.
   It can also prevents one initialized by usb_sg_init from starting,
   so that call just frees resources allocated to the request.