iio: potentiometer: mcp4131: 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: 22d199a539 ("iio: potentiometer: add driver for Microchip MCP413X/414X/415X/416X/423X/424X/425X/426X")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220508175712.647246-85-jic23@kernel.org

drivers/iio/potentiometer/mcp4131.c

@@ -129,7 +129,7 @@ struct mcp4131_data {
 	struct spi_device *spi;
 	const struct mcp4131_cfg *cfg;
 	struct mutex lock;
-	u8 buf[2] ____cacheline_aligned;
+	u8 buf[2] __aligned(IIO_DMA_MINALIGN);
 };
 
 #define MCP4131_CHANNEL(ch) {					\