usb: queue can have async packets

This can happen today in case the ->complete() callback queues up the
next packet.  Also we'll support pipelining soon, which allows to have
multiple packets per queue in flight (aka ASYNC) state.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb.c

@@ -356,6 +356,9 @@ void usb_packet_complete(USBDevice *dev, USBPacket *p)
 
     while (!QTAILQ_EMPTY(&ep->queue)) {
         p = QTAILQ_FIRST(&ep->queue);
+        if (p->state == USB_PACKET_ASYNC) {
+            break;
+        }
         assert(p->state == USB_PACKET_QUEUED);
         ret = usb_process_one(p);
         if (ret == USB_RET_ASYNC) {