net: stmmac: dwmac-meson8b: Fix signedness bug in probe

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

Fixes: 566e8251 ("net: stmmac: add a glue driver for the Amlogic Meson 8b / GXBB DWMAC")
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: NMartin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/dwmac-meson8b.c

@@ -339,7 +339,7 @@ static int meson8b_dwmac_probe(struct platform_device *pdev)
 
 	dwmac->dev = &pdev->dev;
 	dwmac->phy_mode = of_get_phy_mode(pdev->dev.of_node);
-	if (dwmac->phy_mode < 0) {
+	if ((int)dwmac->phy_mode < 0) {
 		dev_err(&pdev->dev, "missing phy-mode property\n");
 		ret = -EINVAL;
 		goto err_remove_config_dt;