net: axienet: fix a signedness bug in probe

The "lp->phy_mode" is an enum but in this context GCC treats it as an
unsigned int so the error handling is never triggered.

Fixes: ee06b1728b ("net: axienet: add support for standard phy-mode binding")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/xilinx/xilinx_axienet_main.c

@@ -1762,7 +1762,7 @@ static int axienet_probe(struct platform_device *pdev)
 		}
 	} else {
 		lp->phy_mode = of_get_phy_mode(pdev->dev.of_node);
-		if (lp->phy_mode < 0) {
+		if ((int)lp->phy_mode < 0) {
 			ret = -EINVAL;
 			goto free_netdev;
 		}