PM / Domains: Fix link checking when add subdomain

Current pm_genpd_add_subdomain() will allow duplicated link between
master and slave domain.  This patch fixed it.

Because when current pm_genpd_add_subdomain() checks whether the link
between the master and slave generic PM domain already exists,
slave_links instead of master_links of master domain is used.

Signed-off-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

drivers/base/power/domain.c

@@ -1448,7 +1448,7 @@ int pm_genpd_add_subdomain(struct generic_pm_domain *genpd,
 		goto out;
 	}
 
-	list_for_each_entry(link, &genpd->slave_links, slave_node) {
+	list_for_each_entry(link, &genpd->master_links, master_node) {
 		if (link->slave == subdomain && link->master == genpd) {
 			ret = -EINVAL;
 			goto out;