Fix authentication for miners with no ethernet about hm-diag

[pritamghanghas]

@shawaj There is another issue with the mac address as password. Our thermal labels fade away overtime. In such a case, the user has to find his mac from his internet router. Assuming he knows how and where to look for mac of connected devices.

[shawaj]

@pritamghanghas I think finding it from router is probably an acceptable amount of effort. We also include 2 additional spare serial number stickers in the box with the miner, as well as the one attached to the miner itself. Also our tech support can probably help people get it as well.

I'm not sure what other default we could use though? And we need the auth to enable other features.

@KevinWassermann94 @NebraLtd/developers any other thoughts on this?