mwifiex: reduce CPU usage by tracking highest_queued_prio

This patch adds highest_queued_prio to track priority of packets as
they are enqueued so that mwifiex_wmm_get_highest_priolist_ptr()
starts checking at the first level where we have packets, instead of
the highest. The function also lowers priority value to the level
where first packet is found.

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>