MDL-Error in case of OsmocomBB+OpenBTS

[Sylvain Munaut]

The fix Andreas just pushed is what I suspected: A missing 'return'
that made the "ignore" casei, not really ignored.

What seems to happen is:
 - We respond to the assignement a bit fast it seems and OpenBTS
doesn't see the first SABM at all (not in the OpenBTS trace at all)
 - When OpenBTS sees the SABM, it responds with the UA frame
 - OpenBTS doesn't send a new frame until quite a bit later, which
means the UA frame is resent several times by OpenBTS transceiver
application using it's filling table (which is why we see only 1 UA
frame in OpenBTS trace but multiple of them in the OsmocomBB trace)
 - OsmocomBB had a bug where it was supposed to ignore the extraneous
UA frame but instead was dropping the connection because of a missing
"return" inside the switch statement.

Cheers,

    Sylvain