ath9k: add reset for airtime station debugfs

Let user can reset station airtime status by debugfs, it will
reset all airtime deficit to ATH_AIRTIME_QUANTUM and reset rx/tx
airtime accumulate to 0.

Signed-off-by: Louie Lu <git@louie.lu>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>