staging: iio: frequency: ad9832: 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.  Whilst here, move the marking to cover
the whole union. That has no functional affect, but makes it slightly
easier to see what is going on.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220813160600.1157169-1-jic23@kernel.org

drivers/staging/iio/frequency/ad9832.c

@@ -112,10 +112,10 @@ struct ad9832_state {
 	 * transfer buffers to live in their own cache lines.
 	 */
 	union {
-		__be16			freq_data[4]____cacheline_aligned;
+		__be16			freq_data[4];
 		__be16			phase_data[2];
 		__be16			data;
-	};
+	} __aligned(IIO_DMA_MINALIGN);
 };
 
 static unsigned long ad9832_calc_freqreg(unsigned long mclk, unsigned long fout)