PCI/ACPI: Request _OSC control before scanning PCI root bus

This patch moves up the code block to request _OSC control in order to
separate ACPI work and PCI work in acpi_pci_root_add().

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