mwifiex: CPU mips optimization with NO_PKT_PRIO_TID

If we go through the entire for loop in
mwifiex_wmm_get_highest_priolist_ptr() and don't find any packets,
set highest_queued_prio to NO_PKT_PRIO_TID (< LOW_PRIO_TID).

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>