wlcore: take the mutex before resetting Tx queues

Otherwise we risk contention for private members of our global structure
while op_stop_locked is running.

Reported-by: Ido Yariv <ido@wizery.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>