ar9170: fix hang on stop

This patch fixes a locking problem which freezes the network core.

The deadlock goes as follows:
- ar9170_op_stop - is called
1. change the state to IDLE
2. > take the MUTEX <
3. cancel_SYNC all pending work, which means
    "block until a work_struct's callback has terminated"
=>  if filter_config_work was queued it tries to get the MUTEX,
       before checking the device state...

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