regulator: fixed: Use gpio_is_valid

Use gpio_is_valid instead of an explicit comparison with 0.
Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/fixed.c

@@ -157,7 +157,7 @@ static int reg_fixed_voltage_probe(struct platform_device *pdev)
 
 	drvdata->desc.fixed_uV = config->microvolts;
 
-	if (config->gpio >= 0) {
+	if (gpio_is_valid(config->gpio)) {
 		cfg.ena_gpio = config->gpio;
 		if (pdev->dev.of_node)
 			cfg.ena_gpio_initialized = true;