libmsc: use new smpp34 esm_class definitions

Replace magic numbers by esm_class definitions, which
have been added to latest libsmpp34 in Change-Id
I91afd8b462b8fd3b2c4c5b54f4eeb7ec5b730b65

Change-Id: I6c458690da60c8f3637680efbd718f6e8c6feb4c