thermal: sysfs: Rework the reading of trip point attributes

Rework the _show() callback functions for the trip point temperature,
hysteresis and type attributes to avoid copying the values of struct
thermal_trip fields that they do not use and make them carry out the
same validation checks as the corresponding _store() callback functions.

No intentional functional impact.

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