PCI hotplug: SGI hotplug: do not use hotplug_slot_attr

By the pci slot changes, callbacks of attributes under slot directory
(/sys/bus/pci/slots) had been changed to get the pointer to struct
pci_slot instead of struct hotplug_slot. So the path_show() that
assumes the parameter is a pointer to struct hotplug_slot seems
broken.

Tested-by: Mike Habeck <habeck@sgi.com>
Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>