usb_cdc: Wake usb_bulk_out_task() on a shutdown

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/generic/usb_cdc.c

@@ -475,6 +475,7 @@ void
 usb_shutdown(void)
 {
     usb_notify_bulk_in();
+    usb_notify_bulk_out();
     usb_notify_ep0();
 }
 DECL_SHUTDOWN(usb_shutdown);