The mii_bus API conversion to read_c45() and write_c45() did not cover
the mdio-mux driver before read() and write() were made C22-only.
This broke arch/arm64/boot/dts/freescale/fsl-ls1028a-qds-13bb.dtso.
The -EOPNOTSUPP from mdiobus_c45_read() is transformed by
get_phy_c45_devs_in_pkg() into -EIO, is further propagated to
of_mdiobus_register() and this makes the mdio-mux driver fail to probe
the entire child buses, not just the PHYs that cause access errors.
Fix the regression by introducing special c45 read and write accessors
to mdio-mux which forward the operation to the parent MDIO bus.
Fixes: db1a63aed89c ("net: phy: Remove fallback to old C45 method")
Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>