esp_eth_phy_802_3_reset_hw() documented ESP_ERR_NOT_ALLOWED for the case where
no reset GPIO is configured, but that path returns ESP_OK: ret is set to ESP_OK
at components/esp_eth/src/phy/esp_eth_phy_802_3.c:489 and never reassigned, so
reset_gpio_num < 0 falls through to `return ret` at line 506.
ESP_ERR_NOT_ALLOWED occurred nowhere else under components/esp_eth.
Documentation comment only; no functional change.
