ath9k: Fix TX poll cancelling

In ath9k_stop(), tx_complete_work was being cancelled twice.
This patch fixes it. Also, locking sc->mutex should be done
at the beginning.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>