thunderbolt: Rework capability handling

Organization of the capabilities in switches and ports is not so random
after all. Rework the capability handling functionality so that it
follows how capabilities are organized and provide two new functions
(tb_switch_find_vse_cap() and tb_port_find_cap()) which can be used to
extract capabilities for ports and switches. Then convert the current
users over these.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Yehezkel Bernat <yehezkel.bernat@intel.com>
Reviewed-by: Michael Jamet <michael.jamet@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andreas Noever <andreas.noever@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>