ar9170: fix lockdep warning on hibernate

This patch takes care of Johannes' deadlock report by moving the
mutex_lock right after cancel_work_sync in ar9170_op_stop.

Besides, the janitor does not need to hold the mutex anymore,
so this extra lines can be removed as well.

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