power_supply: Drop usage of nowarn variant of sysfs_create_link()

The function is not exported to modules, plus we do want to catch anyone
who tries to create complex hierarchy (in that case we'd need to change
'powers' symlink to a directory, probably under a different name to not
break ABI).

This patch fixes the following build error:

 ERROR: "sysfs_create_link_nowarn" [drivers/power/power_supply.ko] undefined!
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>

drivers/power/power_supply_core.c

@@ -159,7 +159,7 @@ EXPORT_SYMBOL_GPL(power_supply_get_by_name);
 
 int power_supply_powers(struct power_supply *psy, struct device *dev)
 {
-	return sysfs_create_link_nowarn(&psy->dev->kobj, &dev->kobj, "powers");
+	return sysfs_create_link(&psy->dev->kobj, &dev->kobj, "powers");
 }
 EXPORT_SYMBOL_GPL(power_supply_powers);