iio: adc: rtq6056: Correct the sign bit index

The vshunt/current reported register is a signed 16bit integer. The
sign bit index should be '15', not '16'.

Fixes: 4396f45 ("iio: adc: Add rtq6056 support")
Reported-by: Andy Hsu <andy_ya_hsu@wiwynn.com>
Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: ChiYuan Huang

drivers/iio/adc/rtq6056.c

@@ -300,7 +300,7 @@ static int rtq6056_adc_read_channel(struct rtq6056_priv *priv,
 		return IIO_VAL_INT;
 	case RTQ6056_REG_SHUNTVOLT:
 	case RTQ6056_REG_CURRENT:
-		*val = sign_extend32(regval, 16);
+		*val = sign_extend32(regval, 15);
 		return IIO_VAL_INT;
 	default:
 		return -EINVAL;