usb-uhci: stop queue filling when we find a in-flight td

Not only QHs can form rings, but TDs too.  With the new
queuing/pipelining support we are following TD chains and
can actually walk in circles.  An assert() prevents us from
entering an endless loop then.

Fix is easy:  Just stop queuing when we figure the TD we are
about to queue up is in flight already.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/hcd-uhci.c

@@ -965,6 +965,9 @@ static void uhci_fill_queue(UHCIState *s, UHCI_TD *td)
         }
         trace_usb_uhci_td_queue(plink & ~0xf, ptd.ctrl, ptd.token);
         ret = uhci_handle_td(s, plink, &ptd, &int_mask);
+        if (ret == TD_RESULT_ASYNC_CONT) {
+            break;
+        }
         assert(ret == TD_RESULT_ASYNC_START);
         assert(int_mask == 0);
         plink = ptd.link;