s6gmac: Read buffer overflow

Check whether index is within bounds before testing the element.
In the last iteration i is PHY_MAX_ADDR. the condition
`!(p = pd->mii.bus->phy_map[PHY_MAX_ADDR])' is undefined and may
evaluate to false, which leads to a dereference of this invalid
phy_map in the phy_connect() below.
Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/s6gmac.c

@@ -793,7 +793,7 @@ static inline int s6gmac_phy_start(struct net_device *dev)
 	struct s6gmac *pd = netdev_priv(dev);
 	int i = 0;
 	struct phy_device *p = NULL;
-	while ((!(p = pd->mii.bus->phy_map[i])) && (i < PHY_MAX_ADDR))
+	while ((i < PHY_MAX_ADDR) && (!(p = pd->mii.bus->phy_map[i])))
 		i++;
 	p = phy_connect(dev, dev_name(&p->dev), &s6gmac_adjust_link, 0,
 			PHY_INTERFACE_MODE_RGMII);