[usb] Allow usb_stream() to enforce a terminating short packet

[usb] Allow usb_stream() to enforce a terminating short packet

Some USB endpoints require that a short packet be used to terminate
transfers, since they have no other way to determine message
boundaries.  If the message length happens to be an exact multiple of
the USB packet size, then this requires the use of an additional
zero-length packet.

Signed-off-by: Michael Brown <mcb30@ipxe.org>