Refactor the code and rename the last passive trip point field.

There is a comment describing the field properly. Use shorter field name
so as to allow to clarify the code.

This change is not expected to alter the general functionality.
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>