PCI: pciehp: remove num_slots field

Since PCIe downstream port has only one slot at most, we don't need
num_slots field in struct controller. Note that struct controller
itself doesn't exist if PCIe downstream port has no slot.

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>