net: dsa: mv88e6xxx: move call to mv88e6xxx_mdios_register()

Call the rather expensive mv88e6xxx_mdios_register() at the beginning of
mv88e6xxx_setup(). This avoids the double call via mv88e6xxx_probe()
during boot.

For symmetry, call mv88e6xxx_mdios_unregister() at the end of
mv88e6xxx_teardown().

Link: https://lore.kernel.org/lkml/449bde236c08d5ab5e54abd73b645d8b29955894.camel@gmail.com/


Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Klaus Kudielka <klaus.kudielka@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Tested-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>