realtek: 5.15: fix warning for i2c_mii_valid_phy_id and i2c_mii_phy_addr

realtek: 5.15: fix warning for i2c_mii_valid_phy_id and i2c_mii_phy_addr

Rework exposing i2c_mii_valid_phy_id and i2c_mii_phy_addr in global
include.
Fix compilation warning:
In file included from drivers/net/phy/sfp.c:11:
./include/linux/mdio/mdio-i2c.h:27:21: error: 'i2c_mii_phy_addr' defined but not used [-Werror=unused-function]
   27 | static unsigned int i2c_mii_phy_addr(int phy_id)
      |                     ^~~~~~~~~~~~~~~~
./include/linux/mdio/mdio-i2c.h:22:13: error: 'i2c_mii_valid_phy_id' defined but not used [-Werror=unused-function]
   22 | static bool i2c_mii_valid_phy_id(int phy_id)
      |             ^~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>