net: sched: ignore tx_queue_len when assigning default qdisc

Since alloc_netdev_mqs() sets IFF_NO_QUEUE for drivers not initializing
tx_queue_len, it is safe to assume that if tx_queue_len is zero,
dev->priv flags always contains IFF_NO_QUEUE.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>