net/mlx5: Re-organize mlx5_cmd struct

Downstream patch will split mlx5_cmd_init() to probe and reload
routines. As a preparation, organize mlx5_cmd struct so that any
field that will be used in the reload routine are grouped at new
nested struct.

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