ACPI / thermal: remove const from thermal_zone_device_ops declaration

The following commit introduced the requirement to not declare
thermal_zone_device_ops structs as const in order to allow
changing the .get_temp callback.

commit 4e5e4705
Author: Eduardo Valentin <eduardo.valentin@ti.com>
Date:   Wed Jul 3 15:35:39 2013 -0400

    thermal: introduce device tree parser

Modify acpi_thermal_zone_ops to follow the new requirement.

Signed-off-by: Emil Goode <emilgoode@gmail.com>
Acked-by: Eduardo Valentin <eduardo.valentin@ti.com>
[rjw: Changelog]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>