iio: adc: modify NPCM ADC read reference voltage

Checking if regulator is valid before reading
NPCM ADC regulator voltage to avoid system crash
in a case the regulator is not valid.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/npcm_adc.c

@@ -149,7 +149,7 @@ static int npcm_adc_read_raw(struct iio_dev *indio_dev,
 		}
 		return IIO_VAL_INT;
 	case IIO_CHAN_INFO_SCALE:
-		if (info->vref) {
+		if (!IS_ERR(info->vref)) {
 			vref_uv = regulator_get_voltage(info->vref);
 			*val = vref_uv / 1000;
 		} else {