ESP8266: Always call ‘wifi_wps_enable’ before ‘wifi_set_wps_cb’

When using the non-OS SDK to enable WPS on the ESP8266, always call wifi_wps_enable() before setting the callback function with wifi_set_wps_cb().

For example:

void wps_start()
{
    wifi_wps_enable(WPS_TYPE_PBC);
    wifi_set_wps_cb(wps_callback);
    wifi_wps_start();
}

In the callback function, call wifi_wps_disable() before wifi_station_connect().

For example:

void wps_callback(int status)
{
    wifi_wps_disable();
    wifi_station_connect();
}