iio: adc: ad4000: fix reading unsigned data

Fix reading unsigned data from the AD4000 ADC via the _raw sysfs
attribute by ensuring that *val is set before returning from
ad4000_single_conversion(). This was not being set in any code path
and was causing the attribute to return a random value.

Fixes: 938fd562 ("iio: adc: Add support for AD4000")
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241030-iio-adc-ad4000-fix-reading-unsigned-data-v1-1-2e28dd75fe29@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>