PCI Hotplug: acpiphp: find bridges the easy way

Instead of constantly evaluating _ADR and _SEG over and over again,
let's simplify our lives by using:

	acpi_pci_find_root() for root bridges
	acpi_get_pci_dev() for p2p bridges

This change eliminates some copy 'n paste code and also allows us
to simplify some internal interfaces.

Reviewed-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Tested-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Cc: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>