Delay ImmAss(PCH, PktDlAss) if waiting for PKT_CTRL_ACK answering UL_ACK_NACK (FinACK=1)

In that state (ul_tbf=TBF_ST_FINISHED), we are unable to reach the MS to
assign a new DL TBF.
* MS Is not available in CCCH because it's attached the PDCH.
* MS won't be able to PKT_CTRL_ACK a PktDlAss on PACCH, because next
  thing it will do is to PKT_CTRL_ACK the UL_ACK_NACK(FINACK=1) we
  already polled it for, and immediatelly after that it will release the
  UL TBF on its side and go back to packet idle mode.

Hence, we must wait for MS to send the PKT_CTRL_ACK to us in order to be
able to assign the DL TBF in PCH (CCCH).

Related: OS#5700
Change-Id: I7a30db9cc7dae70e04054f1a4dba004bd1780d4a
diff --git a/src/tbf.cpp b/src/tbf.cpp
index 4f0162e..d115b82 100644
--- a/src/tbf.cpp
+++ b/src/tbf.cpp
@@ -955,3 +955,8 @@
 {
 	return tbf->establish_dl_tbf_on_pacch();
 }
+
+struct gprs_rlcmac_trx *tbf_get_trx(struct gprs_rlcmac_tbf *tbf)
+{
+	return tbf->trx;
+}