net/mlx5: Open-code create and destroy QP calls

FPGA, IPoIB and SW steering don't need anything from the
mlx5_core_create_qp() and mlx5_core_destroy_qp() except calls
to mlx5_cmd_exec().

Let's open-code it, so we will be able to move qp.c to mlx5_ib.

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