usb-redir: Limit return values returned by iso packets

The usbredir protocol uses a status of usb_redir_stall to indicate that
an iso data stream has stopped (ie because the urbs failed on resubmit),
but iso packets should never return a result of USB_RET_STALL, since iso
endpoints cannot stall. So instead simply always return USB_RET_NAK on
iso stream errors.

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