net/mlx5e: HTB, move stats and max_sqs to priv

Preparation for dynamic allocation of the HTB struct.
The statistics should be preserved even when the struct is de-allocated.

Signed-off-by: Moshe Tal <moshet@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>