Refactor and clarify tbf->triger_ass() code

Split the function into 2 functions, one for assignment on PACCH and one
for assignment on PCH. This makes code calling this API far more clearer
on what is the exact aim when assigning the TBF.

Change-Id: Ic92867e55337b0bd6b5bfc97f13b7982eedb1cb7
diff --git a/src/gprs_ms.c b/src/gprs_ms.c
index 5733a7c..c072b0a 100644
--- a/src/gprs_ms.c
+++ b/src/gprs_ms.c
@@ -1085,7 +1085,10 @@
 	LOGPTBFDL(dl_tbf, LOGL_DEBUG, "[DOWNLINK] START\n");
 
 	/* Trigger the assignment now. */
-	tbf_dl_trigger_ass(dl_tbf, ul_tbf_as_tbf(ul_tbf));
+	if (ul_tbf)
+		dl_tbf_trigger_ass_on_pacch(dl_tbf, ul_tbf_as_tbf(ul_tbf));
+	else
+		dl_tbf_trigger_ass_on_pch(dl_tbf);
 	return 0;
 }