staging: et131x: Remove useless assignment to NULL

The stack variable skb is no longer used after it's set to
NULL. Don't set it to NULL.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/et131x/et131x.c

@@ -4121,7 +4121,6 @@ static netdev_tx_t et131x_tx(struct sk_buff *skb, struct net_device *netdev)
 
 drop_err:
 	dev_kfree_skb_any(skb);
-	skb = NULL;
 	adapter->netdev->stats.tx_dropped++;
 	return NETDEV_TX_OK;
 }