iio: dac: mcp4922: 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.

Fixes: 1b791fad ("iio: dac: mcp4902/mcp4912/mcp4922 dac driver")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Acked-by: Michael Welling <mwelling@ieee.org>
Link: https://lore.kernel.org/r/20220508175712.647246-61-jic23@kernel.org