esp_eth_increase_reference() function

- ESP_OK: increase reference successfully - ESP_ERR_INVALID_ARG: increase reference failed because of some invalid argument

Ethernet driver handle can be obtained by os timer, netif, etc. It's dangerous when thread A is using Ethernet but thread B uninstall the driver. Using reference counter can prevent such risk, but care should be taken, when you obtain Ethernet driver, this API must be invoked so that the driver won't be uninstalled during your using time.