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/gsm_04_08.c b/openbsc/src/gsm_04_08.c
index 521dab4..5eb6a24 100644
--- a/openbsc/src/gsm_04_08.c
+++ b/openbsc/src/gsm_04_08.c
@@ -1330,7 +1330,7 @@
 	if (!msg)
 		return -ENOMEM;
 	memcpy(msg->data, mncc, sizeof(struct gsm_mncc));
-	msgb_enqueue(&net->upqueue, msg);
+	mncc_upq_enqueue(net, msg);
 
 	return 0;
 }
diff --git a/openbsc/src/rtp_proxy.c b/openbsc/src/rtp_proxy.c
index a1f440d..a2a9107 100644
--- a/openbsc/src/rtp_proxy.c
+++ b/openbsc/src/rtp_proxy.c
@@ -443,7 +443,7 @@
 		if (rc < 0)
 			goto out_free;
 		msgb_free(msg);
-		msgb_enqueue(&rs->receive.net->upqueue, new_msg);
+		mncc_upq_enqueue(rs->receive.net, new_msg);
 		break;
 
 	case RTP_NONE: /* if socket exists, but disabled by app */
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;
 	}