iio: amplifiers: ad8366: Change devm_gpiod_get() to optional and add the missed check

Since if there is no GPIO, nothing happens, replace devm_gpiod_get()
with devm_gpiod_get_optional().
Also add IS_ERR() to fix the missing-check warning.

Fixes: cee211f4 ("iio: amplifiers: ad8366: Add support for the ADA4961 DGA")
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Acked-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/amplifiers/ad8366.c

@@ -262,8 +262,11 @@ static int ad8366_probe(struct spi_device *spi)
 	case ID_ADA4961:
 	case ID_ADL5240:
 	case ID_HMC1119:
-		st->reset_gpio = devm_gpiod_get(&spi->dev, "reset",
-			GPIOD_OUT_HIGH);
+		st->reset_gpio = devm_gpiod_get_optional(&spi->dev, "reset", GPIOD_OUT_HIGH);
+		if (IS_ERR(st->reset_gpio)) {
+			ret = PTR_ERR(st->reset_gpio);
+			goto error_disable_reg;
+		}
 		indio_dev->channels = ada4961_channels;
 		indio_dev->num_channels = ARRAY_SIZE(ada4961_channels);
 		break;