[tlan] Guard against failure to identify chip

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/tlan.c

 		}
 		i++;
 	}
+	if (chip_idx == -1)
+		return 0;
 
 	priv->vendor_id = pci->vendor;
 	priv->dev_id = pci->device;