iio: adc: ti-adc084s021: 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.

Update the comment to include 'may'.

Fixes: 3691e5a6 ("iio: adc: add driver for the ti-adc084s021 chip")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Acked-by: Mårten Lindahl <marten.lindahl@axis.com>
Link: https://lore.kernel.org/r/20220508175712.647246-30-jic23@kernel.org

drivers/iio/adc/ti-adc084s021.c

@@ -32,10 +32,10 @@ struct adc084s021 {
 		s64 ts __aligned(8);
 	} scan;
 	/*
-	 * DMA (thus cache coherency maintenance) requires the
+	 * DMA (thus cache coherency maintenance) may require the
 	 * transfer buffers to live in their own cache line.
 	 */
-	u16 tx_buf[4] ____cacheline_aligned;
+	u16 tx_buf[4] __aligned(IIO_DMA_MINALIGN);
 	__be16 rx_buf[5]; /* First 16-bits are trash */
 };