net: dsa: Do not suspend/resume closed slave_dev

If a DSA slave network device was previously disabled, there is no need
to suspend or resume it.

Fixes: 2446254915a7 ("net: dsa: allow switch drivers to implement suspend/resume hooks")
Signed-off-by: Florian Fainelli <[e-mail]>
Reviewed-by: Andrew Lunn <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/dsa/slave.c

···
 {
 	struct dsa_port *dp = dsa_slave_to_port(slave_dev);
 
+	if (!netif_running(slave_dev))
+		return 0;
+
 	netif_device_detach(slave_dev);
 
 	rtnl_lock();
···
 int dsa_slave_resume(struct net_device *slave_dev)
 {
 	struct dsa_port *dp = dsa_slave_to_port(slave_dev);
+
+	if (!netif_running(slave_dev))
+		return 0;
 
 	netif_device_attach(slave_dev);