staging:iio:adc:ad7793 use iio_sw_buffer_preenable to avoid repitition.

Now the generic function caches the scan size a lot of what was here
was redundant and is removed.
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/adc/ad7793.c

@@ -319,31 +319,18 @@ static int ad7793_setup(struct ad7793_state *st)
 static int ad7793_ring_preenable(struct iio_dev *indio_dev)
 {
 	struct ad7793_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 = bitmap_weight(indio_dev->active_scan_mask,
-			       indio_dev->masklength) *
-		indio_dev->channels[0].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);
-
 	st->mode  = (st->mode & ~AD7793_MODE_SEL(-1)) |
 		    AD7793_MODE_SEL(AD7793_MODE_CONT);
 	st->conf  = (st->conf & ~AD7793_CONF_CHAN(-1)) |