PCI/ACPI: Allow _OSC presence to be optional for PCI

The PCI Firmware Spec, r3.2, sec 4.5.1, says:

  For a host bridge device that originates a PCI Express hierarchy, the
  _OSC interface defined in this section is required.  For a host bridge
  device that originates a PCI/PCI-X bus hierarchy, inclusion of an _OSC
  object is optional.

Allow PCI host bridges to bail out silently if _OSC is not found.

Reported-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Sinan Kaya <okaya@kernel.org>
[bhelgaas: cite PCI Firmware spec, the authoritative source]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>