ACPI: Ignore invalid _PSS entries, but use valid ones

The EliteBook 8560W has non-initialized entries in its _PSS ACPI
table. Instead of bailing out when the first non-initialized entry is
found, ignore it and use only  the valid entries. Only bail out if there
is no valid entry at all.

[v3: Fixes suggested by Konrad]

Signed-off-by: Marco Aurelio da Costa <costa@gamic.com>
Signed-off-by: Len Brown <len.brown@intel.com>