dsa: slave: Don't reference NULL pointer during phy_disconnect

When the phy is disconnected, the parent pointer to the netdev it was
attached to is set to NULL. The code then tries to suspend the phy,
but dsa_slave_fixed_link_update needs the parent pointer to determine
which switch the phy is connected to. So it dereferenced a NULL
pointer. Check for this condition.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/slave.c

@@ -896,11 +896,15 @@ static void dsa_slave_adjust_link(struct net_device *dev)
 static int dsa_slave_fixed_link_update(struct net_device *dev,
 				       struct fixed_phy_status *status)
 {
-	struct dsa_slave_priv *p = netdev_priv(dev);
-	struct dsa_switch *ds = p->parent;
+	struct dsa_slave_priv *p;
+	struct dsa_switch *ds;
 
+	if (dev) {
+		p = netdev_priv(dev);
+		ds = p->parent;
 		if (ds->drv->fixed_link_update)
 			ds->drv->fixed_link_update(ds, p->port, status);
+	}
 
 	return 0;
 }