ses: add reliable slot attribute

The name provided by firmware is in a vendor specific format, publish
the slot number to have a reliable mechanism for identifying slots
across firmware implementations.  If the enclosure does not provide a
slot number fallback to the component number which is guaranteed unique,
and usually mirrors the slot number.

Cleaned up the unused ses_component.desc in the process.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Reviewed-by: Jens Axboe <axboe@fb.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>