ath9k: Fix work handling

* Currently, there is no synchronization between the reset
  work and the tx-poll work. Fix this and make sure that we
  bail out properly if a reset work is in progress.

* Cleanup the PLL WAR and enable it for AR9340 too and
  use a helper for restarting work/timers after a reset.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>