PCI: pciehp: remove slot_list field

Since PCIe downstream port has only one slot at most, we don't need
'slot_list' linked list to manage multiple slots under the port.

Acked-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>