kset: convert pci hotplug to use kset_create_and_add

This also renames pci_hotplug_slots_subsys to pcis_hotplug_slots_kset
catch all current users with a build error instead of a build warning
which can easily be missed.

Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Kristen Carlson Accardi <kristen.c.accardi@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>