iio: adc: ad7768-1: Fix conversion result sign

The ad7768-1 ADC output code is two's complement, meaning that the voltage
conversion result is a signed value.. Since the value is a 24 bit one,
stored in a 32 bit variable, the sign should be extended in order to get
the correct representation.

Also the channel description has been updated to signed representation,
to match the ADC specifications.

Fixes: a5f8c7d ("iio: adc: Add AD7768-1 ADC basic support")
Reviewed-by: David Lechner <[email protected]>
Reviewed-by: Marcelo Schmitt <[email protected]>
Signed-off-by: Sergiu Cuciurean <[email protected]>
Signed-off-by: Jonathan Santos <[email protected]>
Cc: <[email protected]>
Link: https://patch.msgid.link/505994d3b71c2aa38ba714d909a68e021f12124c.1741268122.git.Jonathan.Santos@analog.com
Signed-off-by: Jonathan Cameron <[email protected]>

Author: scuciurean

drivers/iio/adc/ad7768-1.c

@@ -142,7 +142,7 @@ static const struct iio_chan_spec ad7768_channels[] = {
 		.channel = 0,
 		.scan_index = 0,
 		.scan_type = {
-			.sign = 'u',
+			.sign = 's',
 			.realbits = 24,
 			.storagebits = 32,
 			.shift = 8,
@@ -373,7 +373,7 @@ static int ad7768_read_raw(struct iio_dev *indio_dev,
 		iio_device_release_direct(indio_dev);
 		if (ret < 0)
 			return ret;
-		*val = ret;
+		*val = sign_extend32(ret, chan->scan_type.realbits - 1);
 
 		return IIO_VAL_INT;