net: dsa: make slave close symmetrical to open

The DSA slave open function configures the unicast MAC addresses on the
master device, enable the switch port, change its STP state, then start
the PHY device.

Make the close function symmetric, by first stopping the PHY device,
then changing the STP state, disabling the switch port and restore the
master device.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>