thermal/debugfs: Adjust check for trips without statistics in tze_seq_show()

Initialize the trip_temp field in struct trip_stats to
THERMAL_TEMP_INVALID and adjust the check for trips without
statistics in tze_seq_show() to look at that field instead of
comparing min and max.

This will mostly be useful to simplify subsequent changes.

No intentional functional impact.

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