RDMA/mlx5: Add NIC TX namespace when getting a flow table

Add the ability to get a NIC TX flow table when using _get_flow_table().
This will allow to create a matcher and a flow rule on the NIC TX path.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>