RDMA/mlx5: Convert modify QP to use MLX5_SET macros

Instead of hand crafted mlx5_qp_context and mlx5_qp_path use common
MLX5_SET() macros.

Link: https://lore.kernel.org/r/20200526115440.205922-7-leon@kernel.org


Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Mark Zhang <markz@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>