libertas: fix GSPI card event handling

The GPSI interface driver does not re-enable the Card Event Interrupt, which
causes problems after a card event (for example: link-loss) comes in.  This
can lead, for example, to the card failing to re-associate.  This patch
ensures that we re-enable the Card Event Interrupt when we handle card events.

Signed-off-by: Andrey Yurovsky <andrey@cozybit.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>