net/mlx5: split mlx5_cmd_init() to probe and reload routines

There is no need to destroy and allocate cmd SW structs during reload,
this is time consuming for no reason.
Hence, split mlx5_cmd_init() to probe and reload routines.

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