net: fec: normalize return value of pm_runtime_get_sync() in MDIO write

If fec MDIO write method succeeds its return value comes from
call to pm_runtime_get_sync().
But pm_runtime_get_sync() can also return 1.

In case of Micrel KSZ9031 PHY this value will then
be returned along the call chain of phy_write() ->
ksz9031_extended_write() -> ksz9031_center_flp_timing() ->
ksz9031_config_init() -> phy_init_hw() -> phy_attach_direct() ->
phy_connect_direct().

Then phy_connect() will cast it into a pointer using ERR_PTR(),
which then fec_enet_mii_probe() will try to dereference
resulting in an oops.

Fix it by normalizing return value of pm_runtime_get_sync()
to be zero if positive in MDIO write method.

Fixes: 8fff755e9f8d ("net: fec: Ensure clocks are enabled while using mdio bus")
Signed-off-by: Maciej Szmigiero <mail@maciej.szmigiero.name>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/fec_main.c

@@ -1816,11 +1816,13 @@ static int fec_enet_mdio_write(struct mii_bus *bus, int mii_id, int regnum,
 	struct fec_enet_private *fep = bus->priv;
 	struct device *dev = &fep->pdev->dev;
 	unsigned long time_left;
-	int ret = 0;
+	int ret;
 
 	ret = pm_runtime_get_sync(dev);
 	if (ret < 0)
 		return ret;
+	else
+		ret = 0;
 
 	fep->mii_timeout = 0;
 	reinit_completion(&fep->mdio_done);