Stop network queues during Tx flush, and also drop other internal
mac80211 packets (mgmt) that may arrive when the queues are stopped.

When flush is done all driver queues are clear, forcefully if needed.

Protect the Tx flush operation with a new mutex, to prevent concurrency
that can mess us queue state.

Based on a patch by Eliad Peller <eliad@wizery.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>