mei: rename mei_free_cb_private to mei_io_cb_free

1. cb_private was an old name that we depriacated in earlier
cleanups

2. we also group the funcion declaration with other _io_
functions

3. Don't check cb for NULL as mei_io_cb_free is NULL safe

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>