net/mlx5: Provide simplified command interfaces

Many mlx5_cmd_exec() callers are not interested in the output from that
command or have standard in/out structures. Those callers simply allocate
those structure on the stack and use sizeof() to provide in/out arguments.

In this naive approach provide simplified versions of mlx5_cmd_exec().

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