PCI/ACPI: Use acpi_pci_root info rather than looking it up again

This patch changes the function signature of walk_root_bridge().

We can omit _STA, _SEG, and _BBN evaluation by passing not acpi_handle
but acpi_pci_root.  Now that acpi_pci_slot_add() which is the only
caller of walk_root_bridge() gets acpi_pci_root structure, changing
signature of walk_root_bridge() is reasonable.

[bhelgaas: changelog]
Signed-off-by: Taku Izumi <izumi.taku@jp.fujitsu.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>