staging: iio: cdc: ad7746: Use _raw and _scale for temperature channels.

Performing the maths to rescale a 24 bit raw reading within the driver
was resulting in precision losses.  So make that userspace's problem
by exporting the scale and letting the maths be done in userspace with
appropriate precision.  Issue identified using roadtester testing
framework.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220626122938.582107-11-jic23@kernel.org

drivers/staging/iio/cdc/ad7746.c

@@ -178,14 +178,16 @@ static const struct iio_chan_spec ad7746_channels[] = {
 		.type = IIO_TEMP,
 		.indexed = 1,
 		.channel = 0,
-		.info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED),
+		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
+		.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE),
 		.address = TEMP_INT,
 	},
 	[TEMP_EXT] = {
 		.type = IIO_TEMP,
 		.indexed = 1,
 		.channel = 1,
-		.info_mask_separate = BIT(IIO_CHAN_INFO_PROCESSED),
+		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
+		.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE),
 		.address = TEMP_EXT,
 	},
 	[CIN1] = {
@@ -581,18 +583,6 @@ static int ad7746_read_channel(struct iio_dev *indio_dev,
 
 	*val = get_unaligned_be24(data) - 0x800000;
 
-	switch (chan->type) {
-	case IIO_TEMP:
-		/*
-		 * temperature in milli degrees Celsius
-		 * T = ((*val / 2048) - 4096) * 1000
-		 */
-		*val = (*val  * 125) / 256;
-		break;
-	default:
-		break;
-	}
-
 	return 0;
 }
 
@@ -607,7 +597,6 @@ static int ad7746_read_raw(struct iio_dev *indio_dev,
 
 	switch (mask) {
 	case IIO_CHAN_INFO_RAW:
-	case IIO_CHAN_INFO_PROCESSED:
 		mutex_lock(&chip->lock);
 		ret = ad7746_read_channel(indio_dev, chan, val);
 		mutex_unlock(&chip->lock);
@@ -666,6 +655,10 @@ static int ad7746_read_raw(struct iio_dev *indio_dev,
 				*val *= 6;
 			*val2 = 23;
 			return IIO_VAL_FRACTIONAL_LOG2;
+		case IIO_TEMP:
+			*val = 125;
+			*val2 = 8;
+			return IIO_VAL_FRACTIONAL_LOG2;
 		default:
 			return -EINVAL;
 		}