ar9170: cancel led worker properly on exit

"[PATCH 3/4 v2] ar9170: fix LED power state handling" revealed
a bug which can cause a ugly crash.

The delayed worker is canceled before the LED class functions are
unregistered... So, if something manages to update the LEDs
while unregister routine is running the timer could fire _after_ the
module has been unloaded.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>