ath9k: Fix TX queue stuck issue.

commit 86271e46 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>