net/mlx5e: HTB, hide and dynamically allocate mlx5e_htb structure

Move structure mlx5e_htb from the main driver include file "en.h" to be
hidden in qos.c where the qos functionality is implemented, forward
declare it for the rest of the driver and allocate it dynamically upon
user demand only.

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