drivers: fuel_gauge: fix integer comparison in api functions

In the fuel_gauge_get_props and fuel_gauge_set_props functions, an array
len is passed as a size_t but compared against an int, which leads to
C++ compiler warnings when using -Werror=sign-compare. The fix is to use
a size_t for the loop counter.

Signed-off-by: Mikkel Jakobsen <mikkel.aunsbjerg@escolifesciences.com>

Author: aunsbjerg

include/zephyr/drivers/fuel_gauge.h

@@ -296,7 +296,7 @@ static inline int z_impl_fuel_gauge_get_props(const struct device *dev,
 {
 	const struct fuel_gauge_driver_api *api = dev->api;
 
-	for (int i = 0; i < len; i++) {
+	for (size_t i = 0; i < len; i++) {
 		int ret = api->get_property(dev, props[i], vals + i);
 
 		if (ret) {
@@ -349,7 +349,7 @@ static inline int z_impl_fuel_gauge_set_props(const struct device *dev,
 					      fuel_gauge_prop_t *props,
 					      union fuel_gauge_prop_val *vals, size_t len)
 {
-	for (int i = 0; i < len; i++) {
+	for (size_t i = 0; i < len; i++) {
 		int ret = fuel_gauge_set_prop(dev, props[i], vals[i]);
 
 		if (ret) {