net: macb: fix probing of PHY not described in the dt

This patch fixes the case where the PHY isn't described in the device
tree. This is due to the way the MDIO bus is registered in the driver:
whether the PHY is described in the device tree or not, the bus is
registered through of_mdiobus_register. The function masks all the PHYs
and only allow probing the ones described in the device tree. Prior to
the Phylink conversion this was also done but later on in the driver
the MDIO bus was manually scanned to circumvent the fact that the PHY
wasn't described.

This patch fixes it in a proper way, by registering the MDIO bus based
on if the PHY attached to a given interface is described in the device
tree or not.

Fixes: 7897b071 ("net: macb: convert to phylink")
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>