docs(esp_eth): drop unreachable return code from esp_eth_phy_802_3_reset_hw

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.

components/esp_eth/include/esp_eth_phy_802_3.h

@@ -205,7 +205,6 @@ esp_err_t esp_eth_phy_802_3_del(phy_802_3_t *phy_802_3);
  * @param phy_802_3 IEEE 802.3 PHY object infostructure
  * @return
  *      - ESP_OK: reset Ethernet PHY successfully
- *      - ESP_ERR_NOT_ALLOWED: reset GPIO not defined
  */
 esp_err_t esp_eth_phy_802_3_reset_hw(phy_802_3_t *phy_802_3);