thunderbolt: Cache adapter specific capability offset into struct port

The adapter specific capability either is there or not if the port does
not hold an adapter. Instead of always finding it on-demand we read the
offset just once when the port is initialized.

While there we update the struct port documentation to follow kernel-doc
format.

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