mwifiex: reduce CPU usage by tracking tx_pkts_queued

This patch adds tx_pkts_queued to track number of packets being
enqueued & dequeued so that mwifiex_wmm_lists_empty() evaluation
is lightweight.

Signed-off-by: Marc Yang <yangyang@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/mwifiex/11n_aggr.c b/drivers/net/wireless/mwifiex/11n_aggr.c
index d3d5e08..f807447 100644
--- a/drivers/net/wireless/mwifiex/11n_aggr.c
+++ b/drivers/net/wireless/mwifiex/11n_aggr.c
@@ -196,6 +196,8 @@
 		if (skb_src)
 			pra_list->total_pkts_size -= skb_src->len;
 
+		atomic_dec(&priv->wmm.tx_pkts_queued);
+
 		spin_unlock_irqrestore(&priv->wmm.ra_list_spinlock,
 				       ra_list_flags);
 		mwifiex_11n_form_amsdu_pkt(skb_aggr, skb_src, &pad);
@@ -257,6 +259,8 @@
 
 		pra_list->total_pkts_size += skb_aggr->len;
 
+		atomic_inc(&priv->wmm.tx_pkts_queued);
+
 		tx_info_aggr->flags |= MWIFIEX_BUF_FLAG_REQUEUED_PKT;
 		spin_unlock_irqrestore(&priv->wmm.ra_list_spinlock,
 				       ra_list_flags);