nat: Use the mgcp_config_alloc to have defaults initialized

Initialize the net_base properly and benefit from future
config changes but also reset certain defauls that we do
not want at the nat.
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index e5c2c39..9cf3ba3 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -1120,7 +1120,7 @@
 		return -4;
 	}
 
-	nat->mgcp_cfg = talloc_zero(nat, struct mgcp_config);
+	nat->mgcp_cfg = mgcp_config_alloc();
 	if (!nat->mgcp_cfg) {
 		fprintf(stderr, "Failed to allocate MGCP cfg.\n");
 		return -5;