ynl: mark max/mask as private for kdoc

Simon mentioned in another thread that it makes kdoc happy
and Jakub confirms that commit e27cb89a22ad ("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>

tools/net/ynl/ynl-gen-c.py

@@ -2125,6 +2125,7 @@ def render_uapi(family, cw):
 
             if const.get('render-max', False):
                 cw.nl()
+                cw.p('/* private: */')
                 if const['type'] == 'flags':
                     max_name = c_upper(name_pfx + 'mask')
                     max_val = f' = {enum.get_mask()},'