RDMA/mlx5: Update mlx5_ib to use new cmd interface

Reuse newly introduced mlx5_cmd_exec_in() and mlx5_cmd_exec_inout() to
reduce code duplication in mlx5_ib module.

Link: https://lore.kernel.org/r/20200506065513.4668-2-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>