GPRS_Components: use more suitable SAPI for DL TBF assignment

The function that assigns the downlink TBF f_ms_exp_dl_tbf_ass_ccch()
uses SAPI PCU_IF_SAPI_AGCH as default but actually downlink TBFs are
assigned via the PCH. This means we have to put PCU_IF_SAPI_PCH into the
parameter list on every function call, so it makes sense to change the
default to PCU_IF_SAPI_PCH and omit the SAPI when calling the function

Related: OS#5927
Change-Id: I49c59bad0162cb303669f6108201f154918b1db3
diff --git a/pcu/GPRS_Components.ttcn b/pcu/GPRS_Components.ttcn
index 59c99d8..64a39dd 100644
--- a/pcu/GPRS_Components.ttcn
+++ b/pcu/GPRS_Components.ttcn
@@ -628,7 +628,7 @@
 	ms.ul_tbf := f_ultbf_new_from_rr_imm_ass(rr_imm_ass);
 }
 
-function f_ms_exp_dl_tbf_ass_ccch(inout GprsMS ms, template PCUIF_Sapi sapi := PCU_IF_SAPI_AGCH,
+function f_ms_exp_dl_tbf_ass_ccch(inout GprsMS ms, template PCUIF_Sapi sapi := PCU_IF_SAPI_PCH,
 				  template GsmRrMessage t_imm_ass := tr_IMM_TBF_ASS(true, ?, ?),
 				  template (present) TsTrxBtsNum nr := tr_TsTrxBtsNum)
 runs on MS_BTS_IFACE_CT {