thermal: gov_fair_share: Fix dependency on trip points ordering

The computation in the fair share governor's get_trip_level() function
currently works under the assumption that the temperature ordering of
trips[] in a thermal zone is ascending, which need not be the case.

However, get_trip_level() can be made work regardless of whether or not
the trips table is ordered by temperature in any way, so change it
accordingly.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>