thermal/sysfs: Always expose hysteresis attributes

Instead of avoiding to expose the hysteresis attributes of a thermal
zone when its get_trip_hyst() operation is not defined, which is
confusing, expose them always and use the default
thermal_zone_get_trip() function returning 0 hysteresis when that
operation is not present.

The hysteresis of 0 is perfectly valid, so this change should not
introduce any backwards compatibility issues.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/20221003092602.1323944-3-daniel.lezcano@linaro.org