[sis190] Avoid unused variable warning in gcc 4.6

Reported-by: Ralph Giles <giles@thaumas.net>
Tested-by: Ralph Giles <giles@thaumas.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/sis190.c

 {
 	struct sis190_private *tp;
 	struct net_device *dev;
-	void *ioaddr;
 	int rc;
 
 	rc = sis190_init_board(pdev, &dev);
 	pci_set_drvdata(pdev, dev);
 
 	tp = netdev_priv(dev);
-	ioaddr = tp->mmio_addr;
 
 	rc = sis190_get_mac_addr(pdev, dev);
 	if (rc < 0)