r8169: let mdio read functions return -ETIMEDOUT

In case of a timeout currently ~0 is returned. Callers often just check
whether a certain bit is set and therefore may behave incorrectly.
So let's return -ETIMEDOUT in case of a timeout.

r8168_phy_ocp_read is used in r8168g_mdio_read only, therefore we can
apply the same change.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>