+ Return codes:
+ o NETDEV_TX_OK everything ok.
+ o NETDEV_TX_BUSY Cannot transmit packet, try later
+ Usually a bug, means queue start/stop flow control is broken in
+ the driver. Note: the driver must NOT put the skb in its DMA ring.
+ o NETDEV_TX_LOCKED Locking failed, please retry quickly.
+ Only valid when NETIF_F_LLTX is set.