r8169: use r8168d_modify_extpage in rtl8168f_config_eee_phy

Use r8168d_modify_extpage() also in rtl8168f_config_eee_phy() to
simplify the code.

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