iio: adc: ad7923: Fix undersized rx buffer.

Fixes tag is where the max channels became 8, but timestamp space was missing
before that.

Fixes: 851644a6 ("iio: adc: ad7923: Add support for the ad7908/ad7918/ad7928")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Daniel Junho <djunho@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20210501165314.511954-3-jic23@kernel.org
Cc: <Stable@vger.kernel.org>

drivers/iio/adc/ad7923.c

@@ -59,8 +59,10 @@ struct ad7923_state {
 	/*
 	 * DMA (thus cache coherency maintenance) requires the
 	 * transfer buffers to live in their own cache lines.
+	 * Ensure rx_buf can be directly used in iio_push_to_buffers_with_timetamp
+	 * Length = 8 channels + 4 extra for 8 byte timestamp
 	 */
-	__be16				rx_buf[4] ____cacheline_aligned;
+	__be16				rx_buf[12] ____cacheline_aligned;
 	__be16				tx_buf[4];
 };