ath9k: Fix panic on driver load

The device has to be marked as invalid before
registering the ISR. HW initialization takes place
after the ISR has been registered, and the invalid
flag is eventually cleared in the ->stop() callback.

Reported-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>