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>