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/rtp_proxy.c b/openbsc/src/rtp_proxy.c
index 21ec004..8d14447 100644
--- a/openbsc/src/rtp_proxy.c
+++ b/openbsc/src/rtp_proxy.c
@@ -442,7 +442,7 @@
 		if (rc < 0)
 			goto out_free;
 		msgb_free(msg);
-		mncc_upq_enqueue(rs->receive.net, new_msg);
+		cc_tx_to_mncc(rs->receive.net, new_msg);
 		break;
 
 	case RTP_NONE: /* if socket exists, but disabled by app */