Since ath9k reports airtime usage directly using the
ieee80211_report_airtime() callback, it shouldn't also report it using the
tx_time in status. Make sure the field is zeroed before TX status is
reported to avoid spurious airtime being accounted by bits being left over
from earlier uses of the cb.

Fixes: 89cea749 ("ath9k: Switch to mac80211 TXQ scheduling and airtime APIs")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>