thermal: Move get_tz_trend to the internal header

The function is not used any place other than the thermal
directory. It does not make sense to export its definition in the
global header as there is no use of it.

Move the definition to the internal header and allow better
self-encapsulation.

Take the opportunity to add the parameter names to make checkpatch
happy and remove the pointless stubs.
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lore.kernel.org/r/20200402142747.8307-5-daniel.lezcano@linaro.org

drivers/thermal/thermal_core.h

@@ -72,6 +72,8 @@ struct thermal_trip {
 	enum thermal_trip_type type;
 };
 
+int get_tz_trend(struct thermal_zone_device *tz, int trip);
+
 /*
  * This structure is used to describe the behavior of
  * a certain cooling device on a certain trip point

include/linux/thermal.h

@@ -414,7 +414,6 @@ int thermal_zone_get_temp(struct thermal_zone_device *tz, int *temp);
 int thermal_zone_get_slope(struct thermal_zone_device *tz);
 int thermal_zone_get_offset(struct thermal_zone_device *tz);
 
-int get_tz_trend(struct thermal_zone_device *, int);
 struct thermal_instance *get_thermal_instance(struct thermal_zone_device *,
 		struct thermal_cooling_device *, int);
 void thermal_cdev_update(struct thermal_cooling_device *);
@@ -473,8 +472,7 @@ static inline int thermal_zone_get_slope(
 static inline int thermal_zone_get_offset(
 		struct thermal_zone_device *tz)
 { return -ENODEV; }
-static inline int get_tz_trend(struct thermal_zone_device *tz, int trip)
-{ return -ENODEV; }
+
 static inline struct thermal_instance *
 get_thermal_instance(struct thermal_zone_device *tz,
 	struct thermal_cooling_device *cdev, int trip)