mac80211: Retry null data frame for power save.

Even if the null data frame is not acked by the AP, mac80211
goes into power save. This might lead to loss of frames
from the AP.
Prevent this by restarting dynamic_ps_timer when ack is not
received for null data frames.

Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Vivek Natarajan <vnatarajan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/include/net/mac80211.h b/include/net/mac80211.h
index 414d774..314e981 100644
--- a/include/net/mac80211.h
+++ b/include/net/mac80211.h
@@ -945,6 +945,11 @@
  *	Hardware supports Unscheduled Automatic Power Save Delivery
  *	(U-APSD) in managed mode. The mode is configured with
  *	conf_tx() operation.
+ *
+ * @IEEE80211_HW_REPORTS_TX_ACK_STATUS:
+ *	Hardware can provide ack status reports of Tx frames to
+ *	the stack.
+ *
  */
 enum ieee80211_hw_flags {
 	IEEE80211_HW_HAS_RATE_CONTROL			= 1<<0,
@@ -965,6 +970,7 @@
 	IEEE80211_HW_SUPPORTS_STATIC_SMPS		= 1<<15,
 	IEEE80211_HW_SUPPORTS_DYNAMIC_SMPS		= 1<<16,
 	IEEE80211_HW_SUPPORTS_UAPSD			= 1<<17,
+	IEEE80211_HW_REPORTS_TX_ACK_STATUS		= 1<<18,
 };
 
 /**