net: mdio: Move mdiobus_c45_addr() next to users

Now that mdiobus_c45_addr() is only used within the MDIO code during
fallback, move the function next to its only users. This function
should not be used any more in drivers, the c45 helpers should be used
in its place, so hiding it away will prevent any new users from being
added.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>