ath9k: Fix a sparse warning

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

drivers/net/wireless/ath/ath9k/xmit.c

@@ -2037,7 +2037,7 @@ static void ath_tx_processq(struct ath_softc *sc, struct ath_txq *txq)
 	}
 }
 
-void ath_tx_complete_poll_work(struct work_struct *work)
+static void ath_tx_complete_poll_work(struct work_struct *work)
 {
 	struct ath_softc *sc = container_of(work, struct ath_softc,
 			tx_complete_work.work);