ARM: orion: Fix DSA platform device after mvmdio conversion

DSA expects the host_dev pointer to be the device structure associated
with the MDIO bus controller driver. First commit breaking that was
c3a07134

 ("mv643xx_eth: convert to use the Marvell Orion MDIO
driver"), and then, it got completely under the radar for a while.

Reported-by: Frans van de Wiel <fvdw@fvdw.eu>
Fixes: c3a07134

 ("mv643xx_eth: convert to use the Marvell Orion MDIO driver")
CC: stable@vger.kernel.org
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>