iio: adc: ad7766: 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 reflect the fact DMA safety 'may' require
separate cachelines.

Fixes: aa16c6bd ("iio:adc: Add support for AD7766/AD7767")
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/20220508175712.647246-16-jic23@kernel.org

drivers/iio/adc/ad7766.c

@@ -45,13 +45,12 @@ struct ad7766 {
 	struct spi_message msg;
 
 	/*
-	 * DMA (thus cache coherency maintenance) requires the
+	 * DMA (thus cache coherency maintenance) may require the
 	 * transfer buffers to live in their own cache lines.
 	 * Make the buffer large enough for one 24 bit sample and one 64 bit
 	 * aligned 64 bit timestamp.
 	 */
-	unsigned char data[ALIGN(3, sizeof(s64)) + sizeof(s64)]
-			____cacheline_aligned;
+	unsigned char data[ALIGN(3, sizeof(s64)) + sizeof(s64)]	__aligned(IIO_DMA_MINALIGN);
 };
 
 /*