[netdrvr natsemi] fix ring clean

Too much copy&paste in a call to pci_unmap_single.

drivers/net/natsemi.c

@@ -1530,7 +1530,7 @@ static void drain_tx(struct net_device *dev)
 	for (i = 0; i < TX_RING_SIZE; i++) {
 		if (np->tx_skbuff[i]) {
 			pci_unmap_single(np->pci_dev,
-				np->rx_dma[i], np->rx_skbuff[i]->len,
+				np->tx_dma[i], np->tx_skbuff[i]->len,
 				PCI_DMA_TODEVICE);
 			dev_kfree_skb(np->tx_skbuff[i]);
 			np->stats.tx_dropped++;