power: supply: bq24257_charger: mark OF related data as maybe unused

The driver can be compile tested with !CONFIG_OF making certain data
unused:

  drivers/power/supply/bq24257_charger.c:1143:34: error: ‘bq24257_of_match’ defined but not used [-Werror=unused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

drivers/power/supply/bq24257_charger.c

@@ -1140,7 +1140,7 @@ static const struct i2c_device_id bq24257_i2c_ids[] = {
 };
 MODULE_DEVICE_TABLE(i2c, bq24257_i2c_ids);
 
-static const struct of_device_id bq24257_of_match[] = {
+static const struct of_device_id bq24257_of_match[] __maybe_unused = {
 	{ .compatible = "ti,bq24250", },
 	{ .compatible = "ti,bq24251", },
 	{ .compatible = "ti,bq24257", },