libertas: let more than one MAC event through

lbs_mac_event_disconnected() was called once and then never again
upon a hardware MAC event.

The reason was that the driver didn't clean the correct bit in the interrupt
cause register of the chip.

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>