qlcnic: 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: Sucheta Chakraborty <sucheta.chakraborty@qlogic.com>
Signed-off-by: Amit Kumar Salecha <amit.salecha@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>