iio: adc: ina2xx: Clarify size requirement for data buffer

The timestamp is inserted into the buffer after the sample data by
iio_push_to_buffers_with_timestamp, document the space requirement for
the timestamp.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>