To avoid conflicts on receiption of a frame while sending, _esp_wifi_recv_cb should not executed while a frame is in transmission

Overriding ethernet_input function _esp_wifi_recv_cb is only used when the WiFi interface is in connected state.

wifi_station_connect and wifi_stattion_disconnect never produce errors. Thus, error handling can be removed.

Different connection states defined to be able to realize a better connect/reconnect handling

Since complete MAC frames are handled, ETHERNET_MAX_LEN has to be used instead of ETHERNET_DATA_LEN for receive buffer size and length check.

Fixes sporadic blocking of the wifi thread in esp_wifi_recv_cb function under heavy network load conditions when frames are coming in faster than they can be processed. Since esp_wifi_recv_cb function is not executed in interrupt context, the msg_send function used for ISR event can block when the message queue is full. With this change esp_wifi can be flooded with icmpv6 packets of maximum size without any problems over hours.