net: mv643xx_eth: use phy_disconnect instead of phy_detach

Using a separated mdio bus driver with mvmdio, phy_detach on network device
removal will not stop the phy and finally lead to NULL pointer dereference
in mvmdio due to non-existent network device. Use phy_disconnect instead
to properly stop phy device from accessing network device prior removal of
the network device.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>