void skel_disconnect (struct usb_interface * interface);
Called by the usb core when the device is removed from the system.
This routine guarantees that the driver will not submit any more urbs by clearing dev->udev. It is also supposed to terminate any currently active urbs. Unfortunately, usb_bulk_msg, used in skel_read, does not provide any way to do this. But at least we can cancel an active write.