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>