thermal/debugfs: Compute maximum temperature for mitigation episode as a whole

Notice that the maximum temperature above the trip point must be the
same for all of the trip points involved in a given mitigation episode,
so it need not be computerd for each of them separately.

It is sufficient to compute the maximum temperature for the mitigation
episode as a whole and print it accordingly, so do that.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>