net: sch_generic: react upon IFF_NO_QUEUE flag

Handle IFF_NO_QUEUE as alternative to tx_queue_len being 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>