WPS: Use Config Error 12 to indicate PBC overlap in M2D

If PBC session overlap is detected between button press on the registrar
and M1 is reception, report session overlap with the Config Error
attribute in M2D to the Enrollee.

src/wps/wps_registrar.c

@@ -1305,14 +1305,15 @@ static struct wpabuf * wps_build_m2(struct wps_data *wps)
 static struct wpabuf * wps_build_m2d(struct wps_data *wps)
 {
 	struct wpabuf *msg;
-	u16 err = WPS_CFG_NO_ERROR;
+	u16 err = wps->config_error;
 
 	wpa_printf(MSG_DEBUG, "WPS: Building Message M2D");
 	msg = wpabuf_alloc(1000);
 	if (msg == NULL)
 		return NULL;
 
-	if (wps->wps->ap && wps->wps->ap_setup_locked)
+	if (wps->wps->ap && wps->wps->ap_setup_locked &&
+	    err == WPS_CFG_NO_ERROR)
 		err = WPS_CFG_SETUP_LOCKED;
 
 	if (wps_build_version(msg) ||
@@ -2013,6 +2014,7 @@ static enum wps_process_res wps_process_m1(struct wps_data *wps,
 			wpa_printf(MSG_DEBUG, "WPS: PBC overlap - deny PBC "
 				   "negotiation");
 			wps->state = SEND_M2D;
+			wps->config_error = WPS_CFG_MULTIPLE_PBC_DETECTED;
 			return WPS_CONTINUE;
 		}
 		wps_registrar_add_pbc_session(wps->wps->registrar,