libertas: convert GET_LOG to a direct command

Now uses __lbs_cmd() to get the "log" (it's actually more a snapshot of
various counters, not a sequential log). Besides the "mechanical" convertion
the patch add the following logical changes:

* Removes the priv->logmsg variable, it was only used in one place anyway,
  also don't blindly get the counters when associating. Getting the
  counters then the user asks via WEXT for them is good enought.
* don't set wstats.discard.fragment with log.rxfrag, because the latter is
  a counter for successfully received packets, not for fragmented packets.

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>