net/mlx5: Remove extra indirection while storing QPN

The FPGA, SW steering and IPoIB need to have only QPN from the
mlx5_core_qp struct, so reduce memory footprint by storing QPN
directly.

Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>