iio: dac: ltc2688: fix voltage scale read

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

Fixes: 832cb9ee

 ("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>