MNCC: rename mncc_upq_enqueue() to cc_tx_to_mncc()

The new function name clearly indicates: The Call Control
(TS 04.08, msc-side in OpenBSC) wants to transmit something to the
MNCC interface.
diff --git a/openbsc/src/trau_mux.c b/openbsc/src/trau_mux.c
index 8b8c561..d521a62 100644
--- a/openbsc/src/trau_mux.c
+++ b/openbsc/src/trau_mux.c
@@ -213,7 +213,7 @@
 		}
 		frame->msg_type = GSM_TCHF_FRAME;
 		frame->callref = ue->callref;
-		mncc_upq_enqueue(ue->net, msg);
+		cc_tx_to_mncc(ue->net, msg);
 
 		return 0;
 	}