ASoC: sta350: Use devm_gpiod_get_optional at appropriate place

devm_gpiod_get_optional is equivalent to devm_gpiod_get(), except that
when no GPIO was assigned to the requested function it will return NULL.
This is convenient for drivers that need to handle optional GPIOs.

I just checked the code in commit 34d7c390
("ASoC: improve usage of gpiod API") and found that it should use
devm_gpiod_get_optional rather than devm_gpiod_get here.
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/sta350.c

@@ -1218,7 +1218,7 @@ static int sta350_i2c_probe(struct i2c_client *i2c,
 	if (IS_ERR(sta350->gpiod_nreset))
 		return PTR_ERR(sta350->gpiod_nreset);
 
-	sta350->gpiod_power_down = devm_gpiod_get(dev, "power-down",
+	sta350->gpiod_power_down = devm_gpiod_get_optional(dev, "power-down",
 							   GPIOD_OUT_LOW);
 	if (IS_ERR(sta350->gpiod_power_down))
 		return PTR_ERR(sta350->gpiod_power_down);