ehci: Fixup q->qtdaddr after cancelling an already completed packet

This avoids the q->qtdaddr == p->qtdaddr asserts we have triggering, when
a queue contains multiple completed packages when we cancel the queue.

I triggered this with windows7 + async interrupt endpoint handling (*)
+ not detecting circles in ehci_fill_queue() properly, which makes the qtd
validation in ehci_fill_queue fail, causing cancellation of the queue on every
mouse event ...

*) Which is not going upstream as it will cause loss of interrupt events on
migration.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
