NET: nps_enet: TX done race condition

We need to set tx_skb pointer before send frame.
If we receive interrupt before we set pointer we will try
to free SKB with wrong pointer.
Now we are sure that SKB pointer will never be NULL during
handling TX done and check is removed.

Signed-off-by: Noam Camus <noamc@ezchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>