ath9k: Enable TIM timer interrupt only when needed.

The TIM timer interrupt is enabled even before the ACK of nullqos
is received which is unnecessary.

Also clean up the CONF_PS part of config callback properly for
better readability.

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath/ath9k/xmit.c b/drivers/net/wireless/ath/ath9k/xmit.c
index 3c790a4..f5cbbcb 100644
--- a/drivers/net/wireless/ath/ath9k/xmit.c
+++ b/drivers/net/wireless/ath/ath9k/xmit.c
@@ -2048,10 +2048,9 @@
 		 */
 		if (bf->bf_isnullfunc &&
 		    (ds->ds_txstat.ts_status & ATH9K_TX_ACKED)) {
-			if ((sc->ps_flags & PS_ENABLED)) {
-				sc->ps_enabled = true;
-				ath9k_hw_setrxabort(sc->sc_ah, 1);
-			} else
+			if ((sc->ps_flags & PS_ENABLED))
+				ath9k_enable_ps(sc);
+			else
 				sc->ps_flags |= PS_NULLFUNC_COMPLETED;
 		}