iio: adc: ad7768-1: Fix too small buffer passed to iio_push_to_buffers_with_timestamp()

Add space for the timestamp to be inserted.  Also ensure correct
alignment for passing to iio_push_to_buffers_with_timestamp()

Fixes: a5f8c7da

 ("iio: adc: Add AD7768-1 ADC basic support")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20210501165314.511954-2-jic23@kernel.org
Cc: <Stable@vger.kernel.org>