ynl: mark max/mask as private for kdoc

Simon mentioned in another thread that it makes kdoc happy
and Jakub confirms that commit e27cb89a ("scripts: kernel-doc: support
private / public marking for enums") actually added the needed
support.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20230727163001.3952878-3-sdf@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>