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/main.h b/drivers/net/wireless/mwifiex/main.h
index 672701d..537b40d 100644
--- a/drivers/net/wireless/mwifiex/main.h
+++ b/drivers/net/wireless/mwifiex/main.h
@@ -213,7 +213,8 @@
 	u32 drv_pkt_delay_max;
 	u8 queue_priority[IEEE80211_MAX_QUEUES];
 	u32 user_pri_pkt_tx_ctrl[WMM_HIGHEST_PRIORITY + 1];	/* UP: 0 to 7 */
-
+	/* Number of transmit packets queued */
+	atomic_t tx_pkts_queued;
 };
 
 struct mwifiex_802_11_security {