powercap/drivers/dtpm: Fix root node initialization

The root node is not set to NULL when the dtpm root node is
removed. Consequently, it is not possible to create a new root
as it is already set.

Set the root node to NULL when the last node is removed.
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/powercap/dtpm.c

@@ -207,6 +207,9 @@ int dtpm_release_zone(struct powercap_zone *pcz)
 	if (dtpm->ops)
 		dtpm->ops->release(dtpm);
 
+	if (root == dtpm)
+		root = NULL;
+
 	kfree(dtpm);
 
 	return 0;