fff-wireless: Also use wXmesh label for wXibss

The two different names "wXmesh" and "wXibss" have no functional
purpose and are just for indication. Remove this distinction, so
mesh is just mesh.

This will make further adjustment easier and clearer.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Fabian Bläse <fabian@blaese.de>