PNPACPI: use temporaries to reduce repetition

No functional change, just fewer words and fewer chances for
transcription errors.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Acked-By: Rene Herman <rene.herman@gmail.com>
Signed-off-by: Len Brown <len.brown@intel.com>