net: ethernet: stmmac: Fix signedness bug in ipq806x_gmac_of_parse()

The "gmac->phy_mode" variable is an enum and in this context GCC will
treat it as an unsigned int so the error handling will never be
triggered.

Fixes: b1c17215 ("stmmac: add ipq806x glue layer")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/dwmac-ipq806x.c

@@ -191,7 +191,7 @@ static int ipq806x_gmac_of_parse(struct ipq806x_gmac *gmac)
 	struct device *dev = &gmac->pdev->dev;
 
 	gmac->phy_mode = of_get_phy_mode(dev->of_node);
-	if (gmac->phy_mode < 0) {
+	if ((int)gmac->phy_mode < 0) {
 		dev_err(dev, "missing phy mode property\n");
 		return -EINVAL;
 	}