ath5k: Use DCU early termination correctly

 * DCU early termination should be used to quickly flush QCU
 according to docs so don't enable it for all queues, enable
 it only when stopping each queue and disable it when we are
 done.

 Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>