nat: Attempt to have a single BSC write method

This method currently prepends the IPA header and sends
the data. In the future we might be able to use SCTP for
it.

We have to remove the IPA header from the static messages
for that to work.

This code is untested.
diff --git a/openbsc/src/nat/bsc_mgcp_utils.c b/openbsc/src/nat/bsc_mgcp_utils.c
index 81465ca..a273af4 100644
--- a/openbsc/src/nat/bsc_mgcp_utils.c
+++ b/openbsc/src/nat/bsc_mgcp_utils.c
@@ -177,7 +177,7 @@
 		mgcp_free_endp(mgcp_endp);
 	}
 
-	bsc_write_mgcp_msg(bsc_con, bsc_msg);
+	bsc_write(bsc_con, bsc_msg, NAT_IPAC_PROTO_MGCP);
 	return MGCP_POLICY_DEFER;
 }