net/mlx5e: Timeout if SQ doesn't flush during close

Avoid an infinite loop by timing out waiting for the SQ to flush. Also
clean up the TX descriptors if that happens.

Fixes: f62b8bb8 ('net/mlx5: Extend mlx5_core to support ConnectX-4 Ethernet functionality')
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>