iio: addac: ad74413: fix resistance input processing

On success, ad74413r_get_single_adc_result() returns IIO_VAL_INT aka
1. So currently, the IIO_CHAN_INFO_PROCESSED case is effectively
equivalent to the IIO_CHAN_INFO_RAW case, and we never call
ad74413r_adc_to_resistance_result() to convert the adc measurement to
ohms.

Check ret for being negative rather than non-zero.

Fixes: fea251b6

 (iio: addac: add AD74413R driver)
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20230503095817.452551-1-linux@rasmusvillemoes.dk
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>