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: 977724d20584 ("iio:dac:ti-dac7612: Add driver for Texas Instruments DAC7612")
Signed-off-by: Jonathan Cameron <[e-mail]>
Cc: Ricardo Ribalda <[e-mail]>
Acked-by: Nuno Sá <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/iio/dac/ti-dac7612.c

···
 	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)