thermal: cpu_cooling: return ERR_PTR() for !CPU_THERMAL or !THERMAL_OF

The documentation of of_cpufreq_cooling_register() and
cpufreq_cooling_register() say that they return ERR_PTR() on error.
Accordingly, callers only check for IS_ERR().  Therefore, make them
return ERR_PTR(-ENOSYS) as is customary in the kernel when config
options are missing.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>

include/linux/cpu_cooling.h

@@ -50,7 +50,7 @@ static inline struct thermal_cooling_device *
 of_cpufreq_cooling_register(struct device_node *np,
 			    const struct cpumask *clip_cpus)
 {
-	return NULL;
+	return ERR_PTR(-ENOSYS);
 }
 #endif
 
@@ -65,13 +65,13 @@ unsigned long cpufreq_cooling_get_level(unsigned int cpu, unsigned int freq);
 static inline struct thermal_cooling_device *
 cpufreq_cooling_register(const struct cpumask *clip_cpus)
 {
-	return NULL;
+	return ERR_PTR(-ENOSYS);
 }
 static inline struct thermal_cooling_device *
 of_cpufreq_cooling_register(struct device_node *np,
 			    const struct cpumask *clip_cpus)
 {
-	return NULL;
+	return ERR_PTR(-ENOSYS);
 }
 static inline
 void cpufreq_cooling_unregister(struct thermal_cooling_device *cdev)