Function esp_now_get_user_oui 

pub unsafe extern "C" fn esp_now_get_user_oui(oui: *mut u8) -> i32

@brief Get the OUI (Organization Identifier) in the vendor-specific element for ESPNOW.

@param oui user configured OUI.

@return - ESP_OK : succeed - ESP_ERR_ESPNOW_ARG : invalid argument