thermal/drivers/acerhdf: Make interval setting only at module load time

The thermal zone device structure is in the process of being private
to the thermal framework core code. This driver is directly accessing
and changing the monitoring polling rate.

After discussing with the maintainers of this driver, having the
polling interval at module loading time is enough for their purpose.

Change the code to take into account the interval when the module is
loaded but restrict the permissions so the value can not be changed
afterwards.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Peter Kaestle <peter@piie.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>