stmmac: platform: Fix signedness bug in stmmac_probe_config_dt()

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

Fixes: b9f0b2f6 ("net: stmmac: platform: fix probe for ACPI devices")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_platform.c

@@ -397,6 +397,7 @@ stmmac_probe_config_dt(struct platform_device *pdev, u8 *mac)
 	struct device_node *np = pdev->dev.of_node;
 	struct plat_stmmacenet_data *plat;
 	struct stmmac_dma_cfg *dma_cfg;
+	int phy_mode;
 	void *ret;
 	int rc;
 
@@ -412,10 +413,11 @@ stmmac_probe_config_dt(struct platform_device *pdev, u8 *mac)
 		eth_zero_addr(mac);
 	}
 
-	plat->phy_interface = device_get_phy_mode(&pdev->dev);
-	if (plat->phy_interface < 0)
-		return ERR_PTR(plat->phy_interface);
+	phy_mode = device_get_phy_mode(&pdev->dev);
+	if (phy_mode < 0)
+		return ERR_PTR(phy_mode);
 
+	plat->phy_interface = phy_mode;
 	plat->interface = stmmac_of_get_mac_mode(np);
 	if (plat->interface < 0)
 		plat->interface = plat->phy_interface;