iio: dac: ad5686: fix bit shift read register

This patch solves the register readback issue with the bit shift. When the
dac resolution was lower than the register size (ex. 12 bits out of 16
bits) the readback value was not shifted with the difference in bits and
the value was higher. Also a mask is applied on the read value in order to
get the value relative to the actual bit size.

Fixes: 0357e488b8 ("iio:dac:ad5686: Refactor the driver")
Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/dac/ad5686.c

@@ -128,7 +128,8 @@ static int ad5686_read_raw(struct iio_dev *indio_dev,
 		mutex_unlock(&indio_dev->mlock);
 		if (ret < 0)
 			return ret;
-		*val = ret;
+		*val = (ret >> chan->scan_type.shift) &
+			GENMASK(chan->scan_type.realbits - 1, 0);
 		return IIO_VAL_INT;
 	case IIO_CHAN_INFO_SCALE:
 		*val = st->vref_mv;