power_supply: Fix sparse non static symbol warning

Fixes the following sparse warnings:

drivers/power/power_supply_core.c:540:5: warning:
 symbol '__power_supply_register' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Sebastian Reichel <sre@kernel.org>

drivers/power/power_supply_core.c

@@ -537,7 +537,8 @@ static void psy_unregister_cooler(struct power_supply *psy)
 }
 #endif
 
-int __power_supply_register(struct device *parent, struct power_supply *psy, bool ws)
+static int __power_supply_register(struct device *parent,
+				   struct power_supply *psy, bool ws)
 {
 	struct device *dev;
 	int rc;