net/mlx5: cmdif, Add new api for command execution

Add mlx5_cmd_do. Unlike mlx5_cmd_exec, this function will not modify
or translate outbox.status.

The function will return:

return = 0: Command was executed, outbox.status == MLX5_CMD_STAT_OK.

return = -EREMOTEIO: Executed, outbox.status != MLX5_CMD_STAT_OK.

return < 0: Command execution couldn't be performed by FW or driver.

And document other mlx5_cmd_exec functions.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>