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

netif_set_real_num_tx/rx_queues() 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_resume()
|- cpsw_ndo_open()
  |- netif_set_real_num_tx/rx_queues()
     |- ASSERT_RTNL();

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

Cc: Dave Gerlach <d-gerlach@ti.com>
Cc: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Fixes: commit e05107e6

 ("net: ethernet: ti: cpsw: add multi queue support")
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Tested-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>