staging: iio: ad7780: fix offset read value

Variable val subtracted an uninitialized value on IIO_CHAN_INFO_OFFSET.
This was fixed by assigning the correct value instead.
Signed-off-by: NRenato Lui Geh <renatogeh@gmail.com>
Acked-by: NAlexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/staging/iio/adc/ad7780.c

@@ -96,7 +96,7 @@ static int ad7780_read_raw(struct iio_dev *indio_dev,
 		*val2 = chan->scan_type.realbits - 1;
 		return IIO_VAL_FRACTIONAL_LOG2;
 	case IIO_CHAN_INFO_OFFSET:
-		*val -= (1 << (chan->scan_type.realbits - 1));
+		*val = -(1 << (chan->scan_type.realbits - 1));
 		return IIO_VAL_INT;
 	}