usb-redir: Never return USB_RET_NAK for async handled packets

USB_RET_NAK is not a valid response for async handled packets (and will
trigger an assert as such).

Also drop the warning when receiving a status of cancelled for packets not
cancelled by qemu itself, this can happen when a device gets unredirected
by the usbredir-host while transfers are pending.

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