iio: Mark iio_dev::priv member with __private

The member is not supposed to be accessed directly, mark it with
__private to catch the misuses up.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20241101105342.3645018-1-andriy.shevchenko@linux.intel.com


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