iwlagn: pending frames musn't be incremented if agg is on

During my works on the transport layer I removed code that updated
a local variable (is_agg) that is needed to keep the pending_frames
count up to date. Fix this.

Also, there should be no way to have a packet with TX_CTL_AMPDU set
while the internal aggregation state machine is not in AGG_ON state.
Add a WARN_ON to ensure that.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/iwlwifi/iwl-agn-tx.c b/drivers/net/wireless/iwlwifi/iwl-agn-tx.c
index abb702d..8c0f07f 100644
--- a/drivers/net/wireless/iwlwifi/iwl-agn-tx.c
+++ b/drivers/net/wireless/iwlwifi/iwl-agn-tx.c
@@ -313,6 +313,9 @@
 		iwl_sta_modify_sleep_tx_count(priv, sta_id, 1);
 	}
 
+	if (info->flags & IEEE80211_TX_CTL_AMPDU)
+		is_agg = true;
+
 	/* irqs already disabled/saved above when locking priv->shrd->lock */
 	spin_lock(&priv->shrd->sta_lock);