iio: dac: ti-dac7612: 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.

Updated help text to 'may' require buffers to be in their own cacheline.

Fixes: 977724d2 ("iio:dac:ti-dac7612: Add driver for Texas Instruments DAC7612")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Ricardo Ribalda <ribalda@kernel.org>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220508175712.647246-65-jic23@kernel.org

drivers/iio/dac/ti-dac7612.c

@@ -31,10 +31,10 @@ struct dac7612 {
 	struct mutex lock;
 
 	/*
-	 * DMA (thus cache coherency maintenance) requires the
+	 * DMA (thus cache coherency maintenance) may require the
 	 * transfer buffers to live in their own cache lines.
 	 */
-	uint8_t data[2] ____cacheline_aligned;
+	uint8_t data[2] __aligned(IIO_DMA_MINALIGN);
 };
 
 static int dac7612_cmd_single(struct dac7612 *priv, int channel, u16 val)