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 <[e-mail]>
Cc: Eduardo Valentin <[e-mail]>
Signed-off-by: Javi Merino <[e-mail]>
Signed-off-by: Eduardo Valentin <[e-mail]>

include/linux/cpu_cooling.h

···
 of_cpufreq_cooling_register(struct device_node *np,
 			    const struct cpumask *clip_cpus)
 {
-	return NULL;
+	return ERR_PTR(-ENOSYS);
 }
 #endif
 
···
 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)