ehci: Verify guest does not change the token of inflight qtd-s

This is not allowed, except for clearing active on cancellation, so don't
warn when the new token does not have its active bit set.

This unifies the cancellation path for modified qtd-s, and prepares
ehci_verify_qtd to be used ad an extra check inside
ehci_writeback_async_complete_packet().

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