uan: (fixes #1046) allow UAN to Tx/Rx 6LoWPAN

I don't know when the example did break, but it looks like UAN isn't handling properly the 6LoWPAN protocol number.

No, I know.

!1800 (merged) make 6LoWPAN a bit more pedantic on what it passes to lower layers. Before (wrongly) it was: change the protocol number to something (user defined) or don't change it because it's unused anyway.

This was a wrong assumption, because if the MAC uses a protocol number and the code is not changed, then the packet will be marked as IPv6 (even if it's not IPv6). Passing a 6LoWPAN frame to the MAC as an IPv6 one is, of course, wrong.

Now 6LoWPAN forces its own protocol number regardless - unless the MAC doesn't have one (only LrWpan), and then it uses 0.

This made UAN crazy, because it didn't know what 6LoWPAN is. This patch fixes that.