thunderbolt: Scan only valid NULL adapter ports in hotplug

The only way to expand Thunderbolt topology is through the NULL adapter
ports (typically ports 1, 2, 3 and 4). There is no point handling
Thunderbolt hotplug events on any other port.

Add a helper function (tb_port_is_null()) that can be used to determine
if the port is NULL port, and use it in software connection manager code
when hotplug event is handled.

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