ath9k: fix race conditions when stop device

We do not kill any scheduled tasklets when stopping device, that may
cause usage of resources after free. Moreover we enable interrupts
in tasklet function, so we could potentially end with interrupts
enabled when driver is not ready to receive them.

I think patch should fix Ben's kernel crash from:
http://marc.info/?l=linux-wireless&m=129438358921501&w=2

Cc: stable@kernel.org
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>