thunderbolt: Add clarifying comments about USB4 terms router and adapter

USB4 spec talks about routers and adapters whereas Thunderbolt 1-3
talked about CIO (Converged I/O) switches and ports. These are the same
thing but might cause confusion so add clarifying comments to struct
tb_switch and struct tb_port about the USB4 terms.

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