libertas: fix compact flash interrupt handling

The old code misbehaved because it polled card status and always called the
"tx over" code-path.

This also fixes a hard lockup by not allowing and card interrupts while
transferring a TX frame or a command into the card.

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>