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/tbf_fsm.c b/src/tbf_fsm.c
index eeccd6b..f019214 100644
--- a/src/tbf_fsm.c
+++ b/src/tbf_fsm.c
@@ -276,7 +276,7 @@
 		 * back to packet-idle mode then we can assign the DL TBF on PCH
 		 * now. */
 		if (!new_ul_tbf_requested && ms_need_dl_tbf(ms))
-			ms_new_dl_tbf_assignment(ms);
+			ms_new_dl_tbf_assigned_on_pch(ms);
 		ms_unref(ms);
 		break;
 	case TBF_EV_MAX_N3103: