ACPI: acpi_pci_set_power_state() cleanups

Minor cleanups to acpi_pci_set_power_state():  use the ACPI and PCI
state symbols to make clear that a mapping is being done between PCI
and ACPI states, instead of using magic numbers.  For paranoia's sake,
report any errors.  Save five bytes (x86_64) too.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Len Brown <len.brown@intel.com>