iio:ad5764: Report scale as fractional value

Move the complexity of calculating the fixed point scale to the core.

Also fix a off by one error in the comment describing the transfer function.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/dac/ad5764.c

@@ -217,7 +217,6 @@ static int ad5764_read_raw(struct iio_dev *indio_dev,
 	struct iio_chan_spec const *chan, int *val, int *val2, long info)
 {
 	struct ad5764_state *st = iio_priv(indio_dev);
-	unsigned long scale_uv;
 	unsigned int reg;
 	int vref;
 	int ret;
@@ -245,15 +244,14 @@ static int ad5764_read_raw(struct iio_dev *indio_dev,
 		*val = sign_extend32(*val, 5);
 		return IIO_VAL_INT;
 	case IIO_CHAN_INFO_SCALE:
-		/* vout = 4 * vref + ((dac_code / 65535) - 0.5) */
+		/* vout = 4 * vref + ((dac_code / 65536) - 0.5) */
 		vref = ad5764_get_channel_vref(st, chan->channel);
 		if (vref < 0)
 			return vref;
 
-		scale_uv = (vref * 4 * 100) >> chan->scan_type.realbits;
-		*val = scale_uv / 100000;
-		*val2 = (scale_uv % 100000) * 10;
-		return IIO_VAL_INT_PLUS_MICRO;
+		*val = vref * 4 / 1000;
+		*val2 = chan->scan_type.realbits;
+		return IIO_VAL_FRACTIONAL_LOG2;
 	case IIO_CHAN_INFO_OFFSET:
 		*val = -(1 << chan->scan_type.realbits) / 2;
 		return IIO_VAL_INT;