net: ethernet: ti: cpsw: fix ASSERT_RTNL() warning during suspend

vlan_for_each() are required to be called with rtnl_lock taken, otherwise
ASSERT_RTNL() warning will be triggered - which happens now during System
resume from suspend:
  cpsw_suspend()
  |- cpsw_ndo_stop()
    |- __hw_addr_ref_unsync_dev()
      |- cpsw_purge_all_mc()
         |- vlan_for_each()
            |- ASSERT_RTNL();

Hence, fix it by surrounding cpsw_ndo_stop() by rtnl_lock/unlock() calls.

Fixes: 15180eca ("net: ethernet: ti: cpsw: fix vlan mcast")
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/ti/cpsw.c

@@ -1753,11 +1753,15 @@ static int cpsw_suspend(struct device *dev)
 	struct cpsw_common *cpsw = dev_get_drvdata(dev);
 	int i;
 
+	rtnl_lock();
+
 	for (i = 0; i < cpsw->data.slaves; i++)
 		if (cpsw->slaves[i].ndev)
 			if (netif_running(cpsw->slaves[i].ndev))
 				cpsw_ndo_stop(cpsw->slaves[i].ndev);
 
+	rtnl_unlock();
+
 	/* Select sleep pin state */
 	pinctrl_pm_select_sleep_state(dev);