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

[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>