r8169: fix page read in r8168g_mdio_read

Functions like phy_modify_paged() read the current page, on Realtek
PHY's this means reading the value of register 0x1f. Add special
handling for reading this register, similar to what we do already
in r8168g_mdio_write(). Currently we read a random value that by
chance seems to be 0 always.

Fixes: a2928d28 ("r8169: use paged versions of phylib MDIO access functions")
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>