net: dsa: bcm_sf2: don't use devres for mdiobus

In the Linux kernel, the following vulnerability has been resolved:

net: dsa: bcm_sf2: don't use devres for mdiobus

As explained in commits:

74b6d7d13307 ("net: dsa: realtek: register the MDIO bus under devres")

5135e96a3dd2 ("net: dsa: don't allocate the slave_mii_bus using devres")

mdiobus_free() will panic when called from devm_mdiobus_free() <-

devres_release_all() <- __device_release_driver(), and that mdiobus was

not previously unregistered.

The Starfighter 2 is a platform device, so the initial set of

constraints that I thought would cause this (I2C or SPI buses which call

->remove on ->shutdown) do not apply. But there is one more which

applies here.

If the DSA master itself is on a bus that calls ->remove from ->shutdown

(like dpaa2-eth, which is on the fsl-mc bus), there is a device link

between the switch and the DSA master, and device_links_unbind_consumers()

will unbind the bcm_sf2 switch driver on shutdown.

So the same treatment must be applied to all DSA switch drivers, which

is: either use devres for both the mdiobus allocation and registration,

or don't use devres at all.

The bcm_sf2 driver has the code structure in place for orderly mdiobus

removal, so just replace devm_mdiobus_alloc() with the non-devres

variant, and add manual free where necessary, to ensure that we don't

let devres free a still-registered bus.