nat: Make the code work in terms of endpoints instead of timeslot/multiplex

We are going to have more than one trunk, so all code hardcoding the
multiplex to zero must go. Avoid this kind of problem by saving the
MGCP endpoint number and comparing that.
diff --git a/openbsc/src/nat/bsc_nat_vty.c b/openbsc/src/nat/bsc_nat_vty.c
index d8af1c2..b72eb4e 100644
--- a/openbsc/src/nat/bsc_nat_vty.c
+++ b/openbsc/src/nat/bsc_nat_vty.c
@@ -121,7 +121,7 @@
 			sccp_src_ref_to_int(&con->patched_ref),
 			con->has_remote_ref,
 			sccp_src_ref_to_int(&con->remote_ref),
-			con->msc_timeslot, con->bsc_timeslot,
+			con->msc_endp, con->bsc_endp,
 			bsc_con_type_to_string(con->con_type),
 			VTY_NEWLINE);
 	}