r8169: switch to phylib functions in rtl_writephy_batch

Switch rtl_writephy_batch() to phylib functions, as a result we can
avoid passing a rtl8169_private parameter.

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