benet: fix the misusage of zero dma address

benet driver wrongly assumes that zero is an invalid dma address
(calls dma_unmap_page for only non zero dma addresses). Zero is a
valid dma address on some architectures. The dma length can be used
here.
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Sathya Perla <sathyap@serverengines.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/benet/be_main.c

@@ -404,7 +404,7 @@ static void unmap_tx_frag(struct pci_dev *pdev, struct be_eth_wrb *wrb,
 	be_dws_le_to_cpu(wrb, sizeof(*wrb));
 
 	dma = (u64)wrb->frag_pa_hi << 32 | (u64)wrb->frag_pa_lo;
-	if (dma != 0) {
+	if (wrb->frag_len) {
 		if (unmap_single)
 			pci_unmap_single(pdev, dma, wrb->frag_len,
 				PCI_DMA_TODEVICE);