libertas: convert CMD_802_11_MAC_ADDRESS to a direct command

* directly call lbs_cmd_with_response()
* only overwrite priv->current_addr once the firmware call succeeded

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>