wifi-scripts: ucode: set default wildcard mac for wifi-station

wifi-scripts: ucode: set default wildcard mac for wifi-station

When creating the PSK file, the old script sets `mac` to
`00:00:00:00:00:00` when `mac` is not specified (see [here][1]),
creating hostapd configuration lines like:

  vlanid=10 00:00:00:00:00:00 MyStrongPassword

That matches any MAC address (a wildcard). The `ucode` script alternative
misses the default, so set it.

[1]: https://github.com/openwrt/openwrt/blob/9c26d144893c25e484ca97c7a3f58cdec6767465/package/network/config/wifi-scripts/files/lib/netifd/hostapd.sh#L428

Signed-off-by: Oldřich Jedlička <[email protected]>
Signed-off-by: Felix Fietkau <[email protected]>