phylib: Make PHYs children of their MDIO bus, not the bus' parent.

commit 18ee49dd ("phylib: rename mii_bus::dev to mii_bus::parent")
changed the parent of PHY devices from the bus to the bus parent.

Then, commit 4dea547f ("phylib: rework to prepare for OF
registration of PHYs") moved the code into phy_device.c

At this point, it is somewhat unclear why the change was seen as
necessary.  But, when we look at the device model tree in
/sys/devices, it is clearly incorrect.  The PHYs should be children of
their MDIO bus.

Change the PHY's parent device to be the MDIO bus device.

Cc: Lennert Buytenhek <buytenh@wantstofly.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>