iio: buffer: cache timestamp offset in scan buffer

iio: buffer: cache timestamp offset in scan buffer

Cache the offset (in bytes) for the timestamp element in a scan buffer.
This will be used later to ensure proper alignment of the timestamp
element in the scan buffer.

The new field could not be placed in struct iio_dev_opaque because we
will need to access it in a static inline function later, so we make it
__private instead. It is only intended to be used by core IIO code.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>