PCI/ACPI: Make acpi_pci_osc_control_set() static

acpi_pci_osc_control_set() is only called from pci_root.c, so stop
exporting it and make it static.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>