mac80211: Send nullfunc frames at lower rate during connection monitor

Recently mac80211 was changed to use nullfunc instead of probe
request for connection monitoring for tx ack status reporting
hardwares. Sometimes in congested network, STA got disconnected
quickly after the association. It was observered that the rate
control was not adopted to environment due to minimal transmission.

As the nullfunc are used for monitoring purpose, these frames should
not be sacrificed for rate control updation. So it is better to send
the monitoring null func frames at minimum rate that could help to
retain the connection.

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

include/net/mac80211.h

@@ -372,6 +372,9 @@ struct ieee80211_bss_conf {
  *	an SP that mac80211 transmits, it is already set; for driver frames
  *	the driver may set this flag. It is also used to do the same for
  *	PS-Poll responses.
+ * @IEEE80211_TX_CTL_USE_MINRATE: This frame will be sent at lowest rate.
+ *	This flag is used to send nullfunc frame at minimum rate when
+ *	the nullfunc is used for connection monitoring purpose.
  *
  * Note: If you have to add new flags to the enumeration, then don't
  *	 forget to update %IEEE80211_TX_TEMPORARY_FLAGS when necessary.
@@ -404,6 +407,7 @@ enum mac80211_tx_control_flags {
 	IEEE80211_TX_INTFL_TKIP_MIC_FAILURE	= BIT(26),
 	IEEE80211_TX_CTL_NO_CCK_RATE		= BIT(27),
 	IEEE80211_TX_STATUS_EOSP		= BIT(28),
+	IEEE80211_TX_CTL_USE_MINRATE		= BIT(29),
 };
 
 #define IEEE80211_TX_CTL_STBC_SHIFT		23