`get_lan()`: If the ethernet MAC address is uninitialised, set it to the
address reserved by the ESP32 for the ETH interface.
SPI LAN devices may be initialised with a MAC address of 00:00:00:00:00:00.
So check that a valid unicast MAC address has been set (using
`LAN.config(mac=...)`) when initialising the LAN interface.
Fixes#15425.
Signed-off-by: Glenn Moloney <glenn.moloney@gmail.com>
288a036253