net/mlx5e: Fix error flow in representor failing to add vport rx rule

On representor init rx error flow the flow steering pointer is being
released so mlx5e_attach_netdev() doesn't have a valid fs pointer
in its error flow. Make sure the pointer is nullified when released
and add a check in mlx5e_fs_cleanup() to verify fs is not null
as representor cleanup callback would be called anyway.

Fixes: af8bbf73 ("net/mlx5e: Convert mlx5e_flow_steering member of mlx5e_priv to pointer")
Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Maor Dickman <maord@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>