PCI: pciehp: remove first_slot field

The slot number can be calculated only by physical slot number field
in the slot capabilities register. So the first_slot field in struct
controller is meaningless and we don't need it.

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>