Replace tbf->establish_dl_tbf_on_pacch() refactoring GprsMs functions

ms_new_dl_tbf_assignment() is split into 2 functions, one to
allocate+assign on PACCH and another one for PCH.
This makes a lot clearer the aim of each caller of the function.
Once this is done, it becomes obvious tbf->establish_dl_tbf_on_pacch()
is basically doing the same as ms_new_dl_tbf_assigned_on_pacch() so drop
it.

Change-Id: I610210e3120c962d91ce8ff94c66161e086761ba
diff --git a/src/pdch.cpp b/src/pdch.cpp
index 1df8db8..ad521e3 100644
--- a/src/pdch.cpp
+++ b/src/pdch.cpp
@@ -421,7 +421,7 @@
 		 * TBF might have been released while the UL TBF has been
 		 * established */
 		if (ms_need_dl_tbf(new_tbf->ms()))
-			new_tbf->establish_dl_tbf_on_pacch();
+			ms_new_dl_tbf_assigned_on_pacch(new_tbf->ms(), new_tbf);
 		return;
 
 	case PDCH_ULC_POLL_DL_ASS: