net/mlx5e: Fix TX Timeout to detect queues stuck on BQL

Change netif_tx_queue_stopped to netif_xmit_stopped.  This will show
when queues are stopped due to byte queue limits.

Fixes: 3947ca18

 ('net/mlx5e: Implement ndo_tx_timeout callback')
Signed-off-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>