Building the Pico-W needs the MICROPY_PY_NETWORK_CYW43 flag to be set in
order to include building the CYW43 Wifi driver. But then mp_hal_get_mac()
handles the MAC assignment for all nics the "CYW43 way", copying the real
MAC provided by the WiFi hardware. This will fail for all other NIC types,
resulting in an invalid MAC address.
The solution in this commit is to add a check for the NIC type parameter
idx and handle the MAC address respectively.
29401a719f