WiFiSTA - allow using DHCP again after disconnecting static IP (esp8266#9020)

Author: JAndrassy

libraries/ESP8266WiFi/src/ESP8266WiFiSTA.cpp

@@ -400,6 +400,8 @@ bool ESP8266WiFiSTAClass::disconnect(bool wifioff, bool eraseCredentials) {
         WiFi.enableSTA(false);
     }
 
+    _useStaticIp = false;
+
     return ret;
 }