thunderbolt: Make tb_path_alloc() work with tree topologies

With USB4, topologies are not limited to daisy-chains anymore so when
calculating how many hops are between two ports we need to walk the
whole path instead.

Add helper function tb_for_each_port_on_path() that can be used to walk
over each port on a path and make tb_path_alloc() to use it.

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