libertas: less eventcause shifts

* only shift eventcause once

* convert mac events to decimal, as this is what the firmware
  manual uses in section 6.1, too

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>