Fix remove_and_add_spares removes drive added as spare in slot_store

Fix remove_and_add_spares removes drive added as spare in slot_store

Commit f2259d9b3f487802ab130971b4b7935f566f62cd
introduced a regression which would remove a recently added spare via
slot_store. Revert part of the patch which touches slot_store() and add
the disk directly using pers->hot_add_disk()

Fixes: f2259d9b3f48 ("md: remove_and_add_spares() to activate specific
rdev")
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Signed-off-by: Pawel Baldysiak <pawel.baldysiak@intel.com>
Signed-off-by: NeilBrown <neilb@suse.com>