r8169: decouple rtl_phy_write_fw from actual driver code

This patch is a further step towards decoupling firmware handling from
the actual driver code. Firmware can be for PHY and/or MAC, and two
pairs of read/write functions are needed for handling PHY firmware and
MAC firmware respectively. Pass these functions via struct rtl_fw and
avoid the ugly switching of mdio_ops behind the back of rtl_writephy().

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