dpp: check that DPP is running in station watch

This was causing unneeded WARNING prints because the DPP state
was never checked. Fix this and bail out if DPP isn't running.

src/dpp.c

@@ -3681,6 +3681,9 @@ static void dpp_station_state_watch(enum station_state state, void *user_data)
 {
 	struct dpp_sm *dpp = user_data;
 
+	if (dpp->state == DPP_STATE_NOTHING)
+		return;
+
 	switch (state) {
 	case STATION_STATE_DISCONNECTED:
 	case STATION_STATE_DISCONNECTING: