thunderbolt: Factor common ICM add and update operations out

The newer ICM will not use link and depth to address devices. Instead it
uses route strings. In order to take advantage of the existing code
factor out common operations so that we can use the same functions with
the new ICM as well.

No functional changes intended.

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