iwlwifi: error checking for number of tfds in queue

When receive reply_tx and ready to decrement the count for number of
tfds in queue, do error checking to prevent error condition and
tfds_in_queue become negative number.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
CC: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/iwlwifi/iwl-tx.c b/drivers/net/wireless/iwlwifi/iwl-tx.c
index 87ce2bd..72136c8 100644
--- a/drivers/net/wireless/iwlwifi/iwl-tx.c
+++ b/drivers/net/wireless/iwlwifi/iwl-tx.c
@@ -120,6 +120,20 @@
 EXPORT_SYMBOL(iwl_txq_update_write_ptr);
 
 
+void iwl_free_tfds_in_queue(struct iwl_priv *priv,
+			    int sta_id, int tid, int freed)
+{
+	if (priv->stations[sta_id].tid[tid].tfds_in_queue >= freed)
+		priv->stations[sta_id].tid[tid].tfds_in_queue -= freed;
+	else {
+		IWL_ERR(priv, "free more than tfds_in_queue (%u:%d)\n",
+			priv->stations[sta_id].tid[tid].tfds_in_queue,
+			freed);
+		priv->stations[sta_id].tid[tid].tfds_in_queue = 0;
+	}
+}
+EXPORT_SYMBOL(iwl_free_tfds_in_queue);
+
 /**
  * iwl_tx_queue_free - Deallocate DMA queue.
  * @txq: Transmit queue to deallocate.
@@ -1559,7 +1573,7 @@
 	if (txq->q.read_ptr != (ba_resp_scd_ssn & 0xff)) {
 		/* calculate mac80211 ampdu sw queue to wake */
 		int freed = iwl_tx_queue_reclaim(priv, scd_flow, index);
-		priv->stations[sta_id].tid[tid].tfds_in_queue -= freed;
+		iwl_free_tfds_in_queue(priv, sta_id, tid, freed);
 
 		if ((iwl_queue_space(&txq->q) > txq->q.low_mark) &&
 		    priv->mac80211_registered &&