staging:iio:meter:ad7758 use iio_sw_buffer_preenable to avoid code repition

Using this generic function adds a little overhead to a slow path but
reduces the amount of code repitition in exchange.
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/iio/meter/ade7758_ring.c

@@ -92,29 +92,19 @@ static irqreturn_t ade7758_trigger_handler(int irq, void *p)
 static int ade7758_ring_preenable(struct iio_dev *indio_dev)
 {
 	struct ade7758_state *st = iio_priv(indio_dev);
-	struct iio_buffer *ring = indio_dev->buffer;
-	size_t d_size;
 	unsigned channel;
+	int ret;
 
 	if (!bitmap_empty(indio_dev->active_scan_mask, indio_dev->masklength))
 		return -EINVAL;
 
+	ret = iio_sw_buffer_preenable(indio_dev);
+	if (ret < 0)
+		return ret;
+
 	channel = find_first_bit(indio_dev->active_scan_mask,
 				 indio_dev->masklength);
 
-	d_size = st->ade7758_ring_channels[channel].scan_type.storagebits / 8;
-
-	if (ring->scan_timestamp) {
-		d_size += sizeof(s64);
-
-		if (d_size % sizeof(s64))
-			d_size += sizeof(s64) - (d_size % sizeof(s64));
-	}
-
-	if (indio_dev->buffer->access->set_bytes_per_datum)
-		indio_dev->buffer->access->
-			set_bytes_per_datum(indio_dev->buffer, d_size);
-
 	ade7758_write_waveform_type(&indio_dev->dev,
 		st->ade7758_ring_channels[channel].address);