net/slip: Update IFF_RUNNING flag by netdev_carrier_on and netdev_carrier_off

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

drivers/net/slip.c

@@ -818,6 +818,7 @@ static int slip_ifup(FAR struct net_driver_s *dev)
   /* Mark the interface up */
 
   priv->bifup = true;
+  netdev_carrier_on(dev);
   return OK;
 }
 
@@ -842,6 +843,8 @@ static int slip_ifdown(FAR struct net_driver_s *dev)
   FAR struct slip_driver_s *priv =
     (FAR struct slip_driver_s *)dev->d_private;
 
+  netdev_carrier_off(dev);
+
   /* Mark the device "down" */
 
   priv->bifup = false;