MNCC: introduce mncc_upq_enqueue() wrapper around msgb_enqueue()

This wrapper will allow us to add queue related code at one single
location in the code as opposed to three locations.
diff --git a/openbsc/src/trau_mux.c b/openbsc/src/trau_mux.c
index f2fa5c0..67af541 100644
--- a/openbsc/src/trau_mux.c
+++ b/openbsc/src/trau_mux.c
@@ -214,7 +214,7 @@
 		}
 		frame->msg_type = GSM_TCHF_FRAME;
 		frame->callref = ue->callref;
-		msgb_enqueue(&ue->net->upqueue, msg);
+		mncc_upq_enqueue(ue->net, msg);
 
 		return 0;
 	}