libertas: allow removal of card at any time

This fixes several problems I had:

* when removing the card while the card was scanning or associtating,
  it could happen that destroy_workqueue() stuck and didn't return.
* make sure the command function doesn't run while we remove the list
  of pending commands
* for still unknown reason, I had calls to lbs_stop_card() with
  priv==NULL

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>