thunderbolt: Fix path indices used in USB3 tunnel discovery

The USB3 discovery used wrong indices when tunnel is discovered. It
should use TB_USB3_PATH_DOWN for path that flows downstream and
TB_USB3_PATH_UP when it flows upstream. This should not affect the
functionality but better to fix it.

Fixes: e6f81858 ("thunderbolt: Add support for USB 3.x tunnels")
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: stable@vger.kernel.org # v5.6+