net/mlx4_en: remove NETDEV_TX_BUSY

Drivers should avoid NETDEV_TX_BUSY as much as possible.

They should stop the tx queue before qdisc even tries to push another
packet, to avoid requeues.

For a driver supporting skb->xmit_more, this is likely to be a prereq
anyway, otherwise we could have a tx deadlock : We need to force a
doorbell if TX ring is full.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>