thermal: allow registering without .get_temp

This patch changes the thermal core driver to allow
registration of thermal zones without the .get_temp callback.

The idea behind this change is to allow lazy registration
of sensor callbacks.

The thermal zone will be disabled whenever the ops
does not contain a .get_temp callback. The sysfs interface
will be returning -EINVAL on any temperature read operation.

Cc: Zhang Rui <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Eduardo Valentin <[e-mail]>

drivers/thermal/thermal_core.c

···
 	enum thermal_trip_type type;
 #endif
 
-	if (!tz || IS_ERR(tz))
+	if (!tz || IS_ERR(tz) || !tz->ops->get_temp)
 		goto exit;
 
 	mutex_lock(&tz->lock);
···
 void thermal_zone_device_update(struct thermal_zone_device *tz)
 {
 	int count;
+
+	if (!tz->ops->get_temp)
+		return;
 
 	update_temperature(tz);
 
···
 	if (trips > THERMAL_MAX_TRIPS || trips < 0 || mask >> trips)
 		return ERR_PTR(-EINVAL);
 
-	if (!ops || !ops->get_temp)
+	if (!ops)
 		return ERR_PTR(-EINVAL);
 
 	if (trips > 0 && (!ops->get_trip_type || !ops->get_trip_temp))
···
 	bind_tz(tz);
 
 	INIT_DELAYED_WORK(&(tz->poll_queue), thermal_zone_device_check);
+
+	if (!tz->ops->get_temp)
+		thermal_zone_device_set_polling(tz, 0);
 
 	thermal_zone_device_update(tz);