iio: core: annotate masklength as __private

iio: core: annotate masklength as __private

Now that all users are using the proper accessors, we can mark
masklength as __private so that no one tries to write. We also get help
from checkers in warning us in case someone does it.

To access the private field from IIO core code, we need to use the
ACCESS_PRIVATE() macro.

Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240726-dev-iio-masklength-private3-v1-23-82913fc0fb87@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>