netxen: fix race in skb->len access

As soon as skb is given to hardware, TX completion can free skb under
us.
Therefore, we should update dev stats before kicking the device.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Amit Kumar Salecha <amit.salecha@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>