iio: dac: ltc2688: fix voltage scale read

Properly set *val2 (and not overwrite *val) to correctly return
IIO_VAL_FRACTIONAL_LOG2.

Fixes: 832cb9e ("iio: dac: add support for ltc2688")
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220412124916.61-1-nuno.sa@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: nunojsa

drivers/iio/dac/ltc2688.c

@@ -298,7 +298,7 @@ static int ltc2688_read_raw(struct iio_dev *indio_dev,
 		if (ret)
 			return ret;
 
-		*val = 16;
+		*val2 = 16;
 		return IIO_VAL_FRACTIONAL_LOG2;
 	case IIO_CHAN_INFO_CALIBBIAS:
 		ret = regmap_read(st->regmap,