thermal/core: fix error code in __thermal_cooling_device_register()

Return an error pointer if ->get_max_state() fails.  The current code
returns NULL which will cause an oops in the callers.

Fixes: c408b3d1 ("thermal: Validate new state in cur_state_store()")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/thermal/thermal_core.c

@@ -896,7 +896,8 @@ __thermal_cooling_device_register(struct device_node *np,
 	cdev->device.class = &thermal_class;
 	cdev->devdata = devdata;
 
-	if (cdev->ops->get_max_state(cdev, &cdev->max_state))
+	ret = cdev->ops->get_max_state(cdev, &cdev->max_state);
+	if (ret)
 		goto out_kfree_type;
 
 	thermal_cooling_device_setup_sysfs(cdev);