staging: octeon: Call SET_NETDEV_DEV()

The Octeon driver calls into PHYLIB which now checks for
net_device->dev.parent, so make sure we do set it before calling into
any MDIO/PHYLIB related function.

Fixes: ec988ad7

 ("phy: Don't increment MDIO bus refcount unless it's a different owner")
Reported-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: stable <stable@vger.kernel.org>  # 4.9+
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>