net/mlx5: Embed mlx5_ttc_table

mlx5_ttc_table struct shouldn't be exposed to the users so
this patch make it internal to ttc.

In addition add a getter function to get the TTC flow table for users
that need to add a rule which points on it.

Signed-off-by: Maor Gottlieb <maorg@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>