ehci: Correct a comment in fetchqtd packet processing

Since my previous comment said "Should never happen", I tried changing the
next line to an assert(0), which did not go well, which as the new comments
explains is logical if you think about it for a moment.
Signed-off-by: NHans de Goede <hdegoede@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/hcd-ehci.c

@@ -2045,7 +2045,10 @@ static int ehci_state_fetchqtd(EHCIQueue *q)
             ehci_set_state(q->ehci, q->async, EST_HORIZONTALQH);
             break;
         case EHCI_ASYNC_FINISHED:
-            /* Should never happen, as this case is caught by fetchqh */
+            /*
+             * We get here when advqueue moves to a packet which is already
+             * finished, which can happen with packets queued up by fill_queue
+             */
             ehci_set_state(q->ehci, q->async, EST_EXECUTING);
             break;
         }