PCI: pciehp: cleanup flow in pciehp_configure_device

Move bus_size_bridges and assign resources out of pciehp_add_bridge()
and do them all together, one time, including slot bridge, to avoid to
calling assign resources several times when there are several bridges
under the slot bridge.  Using pci_assign_unassigned_bridge_resources.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Reviewed-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>