platform/x86/drivers/acerhdf: Check the interval value when it is set

Currently the code checks the interval value when the temperature is
read which is bad for two reasons:

 - checking and setting the interval in the get_temp callback is
   inaccurate and awful, that can be done when changing the value.

 - Changing the thermal zone structure internals is an abuse of the
   exported structure, moreover no lock is taken here.

The goal of this patch is to solve the first item by using the 'set'
function called when changing the interval. The check is done there
and removed from the get_temp function. If the thermal zone was not
initialized yet, the interval is not updated in this case as that will
happen in the init function when registering the thermal zone device.

I don't have any hardware to test the changes.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Peter Kaestle <peter@piie.net>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20201203071738.2363701-2-daniel.lezcano@linaro.org