iio: adc: ad7887: 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'.

Fixes tag is clearly not where this was introduced but it is very unlikely
anyone will back port it past that point.

Fixes: 65dd3d3d ("staging:iio:ad7887: Squash everything into one file")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220508175712.647246-18-jic23@kernel.org

drivers/iio/adc/ad7887.c

@@ -66,13 +66,12 @@ struct ad7887_state {
 	unsigned char			tx_cmd_buf[4];
 
 	/*
-	 * DMA (thus cache coherency maintenance) requires the
+	 * DMA (thus cache coherency maintenance) may require the
 	 * transfer buffers to live in their own cache lines.
 	 * Buffer needs to be large enough to hold two 16 bit samples and a
 	 * 64 bit aligned 64 bit timestamp.
 	 */
-	unsigned char data[ALIGN(4, sizeof(s64)) + sizeof(s64)]
-		____cacheline_aligned;
+	unsigned char data[ALIGN(4, sizeof(s64)) + sizeof(s64)] __aligned(IIO_DMA_MINALIGN);
 };
 
 enum ad7887_supported_device_ids {