LAPDm: Re-structure: Merge Frame type A+B which are really the same

A is B with zero-length payload.  No need to separate them.

Change-Id: I7b3079899f42a61d67be03e3a17346af80d7b3be
diff --git a/library/LAPDm_RAW_PT.ttcn b/library/LAPDm_RAW_PT.ttcn
index d6d3be4..bdfc50b 100644
--- a/library/LAPDm_RAW_PT.ttcn
+++ b/library/LAPDm_RAW_PT.ttcn
@@ -433,7 +433,7 @@
 					lpd.sacch := false;
 				}
 				lpd.sapi := dl.dl_info.link_id.sapi;
-				lpd.lapdm.b := dec_LapdmFrameB(dl.payload.data_ind.payload);
+				lpd.lapdm.ab := dec_LapdmFrameAB(dl.payload.data_ind.payload);
 				LAPDM_SP.send(lpd);
 			}