iio: adc: ad7476: Fix alignment for DMA safety

iio: adc: ad7476: 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 DMA safety 'may' require separate
cachelines.

Fixes tag is unlikely to be the actual introdution of the problem but is
far enough back to cover any likely backporting.

Fixes: 7a28fe3c93d6 ("staging:iio:ad7476: Squash driver into a single file.")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220508175712.647246-14-jic23@kernel.org