thermal/drivers/da9062: Don't access the thermal zone device fields

The driver is reading the passive polling rate in the thermal zone
structure. We want to prevent the drivers to rummage around in the
thermal zone structure.

On the other side, the delay is what the driver passed to the
thermal_zone_device_register() function, so it has already the
information.

Reuse the information we have instead of reading the information we
set.

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