thermal: gov_step_wise: Adjust code logic to match comment

For the algorithm of choosing the next target state in step_wise
governor, the code does the right thing but is implemented in a
way different from what the comment describes. And this hurts the code
readability.

As the logic in the comment is simpler, adjust the code logic to align
with the comment.

No functional change.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
[ rjw: Subject edit ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>