net: dsa: mv88e6xxx: use PHY_DETECT in mac_link_up/mac_link_down

Use the status of the PHY_DETECT bit to determine whether we need to
force the MAC settings in mac_link_up() and mac_link_down().

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>