pm2301-charger: Check return value of regulator_enable

Check the return value of regulator_enable to silence the following
warning:
drivers/power/pm2301_charger.c:725:20: warning:
ignoring return value of ‘regulator_enable’, declared with
attribute warn_unused_result [-Wunused-result]
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>

drivers/power/pm2301_charger.c

@@ -722,8 +722,12 @@ static int pm2xxx_charger_ac_en(struct ux500_charger *charger,
 
 		dev_dbg(pm2->dev, "Enable AC: %dmV %dmA\n", vset, iset);
 		if (!pm2->vddadc_en_ac) {
-			regulator_enable(pm2->regu);
-			pm2->vddadc_en_ac = true;
+			ret = regulator_enable(pm2->regu);
+			if (ret)
+				dev_warn(pm2->dev,
+					"Failed to enable vddadc regulator\n");
+			else
+				pm2->vddadc_en_ac = true;
 		}
 
 		ret = pm2xxx_charging_init(pm2);