mgcp: Remove use of hardwired @mgw from endpoint IDs.

Remove the host portion of the endpoint Id.  This requires less
configuration and we are probably fine to trust that MGCP only
received messages designated for it.
diff --git a/openbsc/src/libmgcp/mgcp_protocol.c b/openbsc/src/libmgcp/mgcp_protocol.c
index 717c1fc..103897c 100644
--- a/openbsc/src/libmgcp/mgcp_protocol.c
+++ b/openbsc/src/libmgcp/mgcp_protocol.c
@@ -431,7 +431,7 @@
 		return find_e1_endpoint(cfg, mgcp);
 	} else {
 		gw = strtoul(mgcp, &endptr, 16);
-		if (gw > 0 && gw < cfg->trunk.number_endpoints && strcmp(endptr, "@mgw") == 0)
+		if (gw > 0 && gw < cfg->trunk.number_endpoints && endptr[0] == '@')
 			return &cfg->trunk.endpoints[gw];
 	}