net: dsa: fix mii_bus to host_dev replacement

dsa_of_probe() still used cd->mii_bus instead of cd->host_dev when
building with CONFIG_OF=y. Fix this by making the replacement here as
well.

Fixes: b4d2394d

 ("dsa: Replace mii_bus with a generic host device")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>