thunderbolt: Introduce tb_port_is_nhi()

This is useful if one needs to check if adapter (port) is the host
interface (NHI). Make tb_port_alloc_hopid() take advantage of this.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>