iio: adc: mcp320x: 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.

Worth noting the fixes tag refers to the same issue being observed
on a platform that probably had only 64 byte cachelines.

Fixes: 0e81bc99 ("iio: mcp320x: Fix occasional incorrect readings")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Michael Welling <mwelling@ieee.org>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220508175712.647246-28-jic23@kernel.org

drivers/iio/adc/mcp320x.c

@@ -92,7 +92,7 @@ struct mcp320x {
 	struct mutex lock;
 	const struct mcp320x_chip_info *chip_info;
 
-	u8 tx_buf ____cacheline_aligned;
+	u8 tx_buf __aligned(IIO_DMA_MINALIGN);
 	u8 rx_buf[4];
 };