ath5k: do not reschedule tx_complete_work on stop

This patch claim to fix "WARNING: at net/mac80211/util.c:599
ieee80211_can_queue_work.isra.7+0x30/0x40", which was reported at:

https://bugzilla.redhat.com/show_bug.cgi?id=922295



We use ATH_STAT_STARTED flag to disallow to perform
ath5k_tx_complete_poll_work() code, hence reschedule
ah->tx_complete_work, when we stop device. This flag was defined in
ath5k code, but it was not used.

I didn't get feedback if the fix works, so patch is compile only tested.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>