iio: core: mark scan_timestamp as __private

Since there are no more direct accesses to the indio_dev->scan_timestamp
value, it can be marked as __private and use the macro ACCESS_PRIVATE()
in order to access it. Like this, static checkers will be able to inform
in case someone tries to either write to the value, or read its value
directly.

Signed-off-by: Vasileios Amoiridis <vassilisamir@gmail.com>
Link: https://patch.msgid.link/20241214191421.94172-5-vassilisamir@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>