chan_iax2: Use OBJ_NODATA to be a bit more explicit.

This is just a minor code cleanup change.  These uses of ao2_callback() would
never return anything since the callbacks always returned 0.  However, be more
explicit that no returned results are wanted by specifying OBJ_NODATA.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@360369 65c4cc65-6c06-0410-ace0-fbb531ad65f3