node: move roam-events to dedicated struct

Move the roam event counters to a dedicated struct.

Update the ubus output to represent this new organization.

Signed-off-by: David Bauer <mail@david-bauer.net>

usteer.h

@@ -88,8 +88,10 @@ struct usteer_node {
 	int max_assoc;
 	int load;
 
-	int roam_source;
-	int roam_destination;
+	struct {
+		int source;
+		int target;
+	} roam_events;
 
 	uint64_t created;
 };