thunderbolt: Add tb_switch_find_by_route()

With the new ICM messaging there is need for find switch by route string
instead of link and depth. Add new function that makes it possible.

Signed-off-by: Radion Mirchevsky <radion.mirchevsky@intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>