power: regulator: common: use dm_gpio_is_valid helper

Use dm_gpio_is_valid() helper function instead of manually checking the
gpio.

Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Tanmay Kathpalia <tanmay.kathpalia@altera.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>

drivers/power/regulator/regulator_common.c

@@ -51,7 +51,7 @@ int regulator_common_get_enable(const struct udevice *dev,
 	struct regulator_common_plat *plat)
 {
 	/* Enable GPIO is optional */
-	if (!plat->gpio.dev)
+	if (!dm_gpio_is_valid(&plat->gpio))
 		return true;
 
 	return dm_gpio_get_value(&plat->gpio);