[xhci] Ensure that zero-length packets are not part of a TRB chain

Some xHCI controllers (such as qemu's emulated xHCI controller) do not
correctly handle zero-length packets that are part of a TRB chain.
The zero-length TRB ends up being squashed and does not result in a
zero-length packet as seen by the device.

Work around this problem by marking the zero-length packet as
belonging to a separate transfer descriptor.

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

src/drivers/usb/xhci.c

@@ -2606,6 +2606,12 @@ static int xhci_endpoint_stream ( struct usb_endpoint *ep,
 		len -= trb_len;
 		trb++;
 	}
+
+	/* Mark zero-length packet (if present) as a separate transfer */
+	if ( zlp && ( count > 1 ) )
+		trb[-2].normal.flags = 0;
+
+	/* Generate completion for final TRB */
 	trb[-1].normal.flags = XHCI_TRB_IOC;
 
 	/* Enqueue TRBs */