tools/lib/atusb.c: eliminated global variable to allow for multiple sessions

- atusb.c: introduced descriptor structure, replacing direct USB device
  handle
- atusb.c (error): moved global variable into descriptor
- driver.h (atusb_dev_handle), atusb.c (atusb_dev_handle), atrf.c
  (atrf_usb_handle): call driver to perform the no longer trivial
  translation

tools/lib/atrf.c

@@ -44,7 +44,7 @@ void *atrf_usb_handle(struct atrf_dsc *dsc)
 {
 #ifdef HAVE_USB
 	if (dsc->driver == &atusb_driver)
-		return dsc->handle;
+		return atusb_dev_handle(dsc->handle);
 #endif
 	return NULL;
 }