ath9k: Fix TX queue stuck issue.

ath9k: Fix TX queue stuck issue.

commit 86271e460a66003dc1f4cbfd845adafb790b7587 introduced a
regression that caused mac80211 queues in stopped state.

ath_drain_all_txq is called in driver flush which would reset
the stopped flag and the mac80211 queues were never started
after that. iperf traffic is completely stalled due to this issue.

Restart the mac80211 queues in driver flush only if the txqs were
drained.

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>