[NET_SCHED]: Reread dev->qdisc for NETDEV_TX_OK

Now that we return the queue length after NETDEV_TX_OK we better
make sure that we have the right queue.  Otherwise we can cause a
stall after a really quick dev_deactive/dev_activate.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>