nat: Start making the number of multiplexes configurable

Introduce a VTY setting that right now needs to be set to
one. To make updating this setting possible we will now store
the number of endpoints in the bsc connection as well.
diff --git a/openbsc/src/nat/bsc_mgcp_utils.c b/openbsc/src/nat/bsc_mgcp_utils.c
index d814f0d..f002926 100644
--- a/openbsc/src/nat/bsc_mgcp_utils.c
+++ b/openbsc/src/nat/bsc_mgcp_utils.c
@@ -48,6 +48,7 @@
 	if (!con->cfg)
 		return -1;
 
+	con->number_endpoints = 31 * con->cfg->number_multiplexes;
 	con->_endpoint_status = talloc_zero_array(con, char,
 						  (32 * con->cfg->number_multiplexes) + 1);
 	return con->_endpoint_status == NULL;
@@ -55,7 +56,7 @@
 
 static int bsc_assign_endpoint(struct bsc_connection *bsc, struct sccp_connections *con)
 {
-	const int number_endpoints = 31 * bsc->cfg->number_multiplexes;
+	const int number_endpoints = bsc->number_endpoints;
 	int i;
 
 	for (i = 1; i <= number_endpoints; ++i) {