of: mdio: Fix a signedness bug in of_phy_get_and_connect()

The "iface" variable is an enum and in this context GCC treats it as
an unsigned int so the error handling is never triggered.

Fixes: b7862412 ("of_mdio: Abstract a general interface for phy connect")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/of/of_mdio.c

@@ -362,7 +362,7 @@ struct phy_device *of_phy_get_and_connect(struct net_device *dev,
 	int ret;
 
 	iface = of_get_phy_mode(np);
-	if (iface < 0)
+	if ((int)iface < 0)
 		return NULL;
 	if (of_phy_is_fixed_link(np)) {
 		ret = of_phy_register_fixed_link(np);