iio: adc: ad7298: Fix alignment for DMA safety

____cacheline_aligned is an insufficient guarantee for non-coherent DMA
on platforms with 128 byte cachelines above L1.  Switch to the updated
IIO_DMA_MINALIGN definition.

Fixes: be7fd3b86ad2 ("iio:adc:ad7298 make the tx and rx buffers __be16")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220508175712.647246-13-jic23@kernel.org

drivers/iio/adc/ad7298.c

@@ -49,7 +49,7 @@ struct ad7298_state {
 	 * DMA (thus cache coherency maintenance) requires the
 	 * transfer buffers to live in their own cache lines.
 	 */
-	__be16				rx_buf[12] ____cacheline_aligned;
+	__be16				rx_buf[12] __aligned(IIO_DMA_MINALIGN);
 	__be16				tx_buf[2];
 };