iio: core: move @scan_index_timestamp to struct iio_dev_opaque

No reason for this cached value to be exposed to drivers so move it
to the opaque structure.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20210426174911.397061-6-jic23@kernel.org