net: phy: factor out provider part from mdio_bus.c

net: phy: factor out provider part from mdio_bus.c

After 52358dd63e34 ("net: phy: remove function stubs") there's a
problem if CONFIG_MDIO_BUS is set, but CONFIG_PHYLIB is not.
mdiobus_scan() uses phylib functions like get_phy_device().
Bringing back the stub wouldn't make much sense, because it would
allow to compile mdiobus_scan(), but the function would be unusable.
The stub returned NULL, and we have the following in mdiobus_scan():

phydev = get_phy_device(bus, addr, c45);
        if (IS_ERR(phydev))
                return phydev;

So calling mdiobus_scan() w/o CONFIG_PHYLIB would cause a crash later in
mdiobus_scan(). In general the PHYLIB functionality isn't optional here.
Consequently, MDIO bus providers depend on PHYLIB.
Therefore factor it out and build it together with the libphy core
modules. In addition make all MDIO bus providers under /drivers/net/mdio
depend on PHYLIB. Same applies to enetc MDIO bus provider. Note that
PHYLIB selects MDIO_DEVRES, therefore we can omit this here.

Fixes: 52358dd63e34 ("net: phy: remove function stubs")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202504270639.mT0lh2o1-lkp@intel.com/
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://patch.msgid.link/c74772a9-dab6-44bf-a657-389df89d85c2@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>