iio: adc: ad7923: 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.

Update the comment to include 'may'.

Note that some other fixes have applied to this line of code
that may complicate automated backporting.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Fixes: 0eac259db2 ("IIO ADC support for AD7923")
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220508175712.647246-19-jic23@kernel.org

drivers/iio/adc/ad7923.c

@@ -57,12 +57,12 @@ struct ad7923_state {
 	unsigned int			settings;
 
 	/*
-	 * DMA (thus cache coherency maintenance) requires the
+	 * DMA (thus cache coherency maintenance) may require the
 	 * transfer buffers to live in their own cache lines.
 	 * Ensure rx_buf can be directly used in iio_push_to_buffers_with_timetamp
 	 * Length = 8 channels + 4 extra for 8 byte timestamp
 	 */
-	__be16				rx_buf[12] ____cacheline_aligned;
+	__be16				rx_buf[12] __aligned(IIO_DMA_MINALIGN);
 	__be16				tx_buf[4];
 };