There's no point probing for phys on this artificial bus, so we can
save a little bit of boot time by telling mdiobus_register() not to do
that.

This doesn't have any functional change, since, at this point,
fixed_mdio_read() returns 0xffff for all addresses/registers, so

  mdiobus_scan() -> get_phy_device() -> get_phy_c22_id()

will return -ENODEV, which is just ignored.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20220606200208.1665417-1-linux@rasmusvillemoes.dkSigned-off-by: Jakub Kicinski <kuba@kernel.org>