staging: wfx: fix unexpected calls to ieee80211_sta_set_buffered()

When a station go to sleep, the driver receive the status REQUEUE and
forward this answer to mac80211. So, neither the driver, neither the
hardware buffer the frames. So the call to ieee80211_sta_set_buffered is
useless.

In add, it seems that mac80211 does not expect to receive
ieee80211_sta_set_buffered(false) after the station is asleep(). When
the device send data to a station, the following sequence can be
observed:

   - Mac80211 call wfx_sta_notify(awake).
   - The driver calls ieee80211_sta_set_buffered(true). Since the
     station is awake, its TIM is not set.
   - Mac80211 receive a power save notification from the station, so it
     calls wfx_sta_notify(asleep).
   - Then, since the driver has declared it has buffered some frames,
     the TIM of the station should be set. This action is delayed by
     mac80211.
   - The device also notice the station go to sleep. It replies the
     REQUEUE status for the buffered frames. The driver forward this
     status to mac80211.
   - There is no more frames in queues, so the driver call
     ieee80211_sta_set_buffered(false).
   - Mac80211 updates the TIM but since there is no frames buffered by
     the driver, it set the TIM for the station to 0.

Anyway, correctly use the ieee80211_sta_set_buffered() API solves the
problem.
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200701150707.222985-8-Jerome.Pouiller@silabs.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/wfx/data_tx.c

@@ -225,7 +225,6 @@ static void wfx_tx_manage_pm(struct wfx_vif *wvif, struct ieee80211_hdr *hdr,
 		sta_priv = (struct wfx_sta_priv *)&sta->drv_priv;
 		spin_lock_bh(&sta_priv->lock);
 		sta_priv->buffered[tid]++;
-		ieee80211_sta_set_buffered(sta, tid, true);
 		spin_unlock_bh(&sta_priv->lock);
 	}
 }
@@ -471,8 +470,6 @@ static void wfx_tx_update_sta(struct wfx_vif *wvif, struct ieee80211_hdr *hdr)
 		spin_lock_bh(&sta_priv->lock);
 		WARN(!sta_priv->buffered[tid], "inconsistent notification");
 		sta_priv->buffered[tid]--;
-		if (!sta_priv->buffered[tid])
-			ieee80211_sta_set_buffered(sta, tid, false);
 		spin_unlock_bh(&sta_priv->lock);
 	} else {
 		dev_dbg(wvif->wdev->dev, "%s: sta does not exist anymore\n",