Using NO_PKT_PRIO_TID and tx_pkts_queued to check for an empty
state, can lead to a contradictory state, resulting in an
infinite loop. Currently queueing and dequeuing of packets is
not synchronized, and can happen concurrently. While tx_pkts_queued
is incremented when adding a packet, max prio is set to NO_PKT when
the WMM list is empty. If a packet is added right after the check
for empty, but before setting max prio to NO_PKT, that packet is
trapped and creates an infinite loop.

Because of the new packet, tx_pkts_queued is at least 1, indicating
wmm lists are not empty. Opposing that max prio is NO_PKT, which
means "skip this wmm queue, it has no packets". The infinite loop
results, because the main loop checks the wmm lists for not empty
via tx_pkts_queued, but for dequeing it uses max_prio to see if it
can skip current list. This will never end, unless a new packet is
added which will restore max prio to the level of the trapped packet.

The solution here is to rely on tx_pkts_queued solely for checking
wmm queue to be empty, and drop the NO_PKT define. It does not
address the locking issue.
Signed-off-by: Andreas Fenkart <andreas.fenkart@streamunlimited.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>