netxen: avoid invalid iounmap

For NX3031 only one I/O range is mapped, so unmapping other
two which are used by older chips, causes this warning on
ppc64.

"Attempt to iounmap early bolted mapping at 0x0000000000000000"
Signed-off-by: Dhananjay Phadke <dhananjay@netxen.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/netxen/netxen_nic_main.c

@@ -1004,7 +1004,9 @@ static void __devexit netxen_nic_remove(struct pci_dev *pdev)
 
 	iounmap(adapter->ahw.db_base);
 	iounmap(adapter->ahw.pci_base0);
+	if (adapter->ahw.pci_base1 != NULL)
 		iounmap(adapter->ahw.pci_base1);
+	if (adapter->ahw.pci_base2 != NULL)
 		iounmap(adapter->ahw.pci_base2);
 
 	pci_release_regions(pdev);