iio: adc: ad7768-1: 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 that separate cachelines 'may' be
required.

Fixes: a5f8c7da ("iio: adc: Add AD7768-1 ADC basic support")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220508175712.647246-17-jic23@kernel.org

drivers/iio/adc/ad7768-1.c

@@ -163,7 +163,7 @@ struct ad7768_state {
 	struct gpio_desc *gpio_sync_in;
 	const char *labels[ARRAY_SIZE(ad7768_channels)];
 	/*
-	 * DMA (thus cache coherency maintenance) requires the
+	 * DMA (thus cache coherency maintenance) may require the
 	 * transfer buffers to live in their own cache lines.
 	 */
 	union {
@@ -173,7 +173,7 @@ struct ad7768_state {
 		} scan;
 		__be32 d32;
 		u8 d8[2];
-	} data ____cacheline_aligned;
+	} data __aligned(IIO_DMA_MINALIGN);
 };
 
 static int ad7768_spi_reg_read(struct ad7768_state *st, unsigned int addr,