net: declare new net_device priv_flag IFF_NO_QUEUE

This private net_device flag can be set by drivers to inform that a
device runs fine without a qdisc attached. This was formerly done by
setting tx_queue_len to zero.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>