power: supply: core: remove opencoded string_lower()

string_lower from string_helpers.h can be used to modify the string
in-place. Use it to get rid of the custom str_to_lower().

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20230929-power_supply-string_lower-v1-1-3141a2634d53@weissschuh.net
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

Author: t-8ch

drivers/power/supply/power_supply_sysfs.c

@@ -15,6 +15,7 @@
 #include <linux/power_supply.h>
 #include <linux/slab.h>
 #include <linux/stat.h>
+#include <linux/string_helpers.h>
 
 #include "power_supply.h"
 
@@ -398,14 +399,6 @@ static const struct attribute_group *power_supply_attr_groups[] = {
 	NULL,
 };
 
-static void str_to_lower(char *str)
-{
-	while (*str) {
-		*str = tolower(*str);
-		str++;
-	}
-}
-
 void power_supply_init_attrs(struct device_type *dev_type)
 {
 	int i;
@@ -420,7 +413,8 @@ void power_supply_init_attrs(struct device_type *dev_type)
 				__func__, i);
 			sprintf(power_supply_attrs[i].attr_name, "_err_%d", i);
 		} else {
-			str_to_lower(power_supply_attrs[i].attr_name);
+			string_lower(power_supply_attrs[i].attr_name,
+				     power_supply_attrs[i].attr_name);
 		}
 
 		attr = &power_supply_attrs[i].dev_attr;