thunderbolt: Introduce tb_port_path_direction_downstream()

Introduce tb_port_path_direction_downstream() to check if path from
source adapter to destination adapter is directed towards downstream.
Convert existing users to call this helper instead of open-coding.

No functional changes.

Signed-off-by: Gil Fine <gil.fine@linux.intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>