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>