net: dsa: move dsa slave destroy code to slave.c

Move dsa slave dedicated code from dsa_switch_destroy to a new
dsa_slave_destroy function in slave.c.
Add the netif_carrier_off and phy_disconnect calls in order to
correctly cleanup the netdev state and PHY state machine.

Signed-off-by: Frode Isaksen <fisaksen@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/dsa/dsa_priv.h b/net/dsa/dsa_priv.h
index 311796c8..1d1a546 100644
--- a/net/dsa/dsa_priv.h
+++ b/net/dsa/dsa_priv.h
@@ -61,6 +61,7 @@
 void dsa_slave_mii_bus_init(struct dsa_switch *ds);
 int dsa_slave_create(struct dsa_switch *ds, struct device *parent,
 		     int port, char *name);
+void dsa_slave_destroy(struct net_device *slave_dev);
 int dsa_slave_suspend(struct net_device *slave_dev);
 int dsa_slave_resume(struct net_device *slave_dev);
 int dsa_slave_netdevice_event(struct notifier_block *unused,