staging: iio: frequency: ad9832: Fix alignment for DMA safety

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