iio: dac: ad5446: Fix read_raw not returning set value

read_raw should return the un-scaled value.

Fixes: 5e06bdfb

 ("staging:iio:dac:ad5446: Return cached value for 'raw' attribute")
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220406105620.1171340-1-michael.hennerich@analog.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>