PCI/ACPI: Drop unnecessary _OSC existence tests

There's no need to check whether _OSC exists here; we eventually
call acpi_evaluate_object(..., "_OSC", ...), and that will fail
gracefully if _OSC doesn't exist.

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