ACPICA: Change handling of PM1 Status register ignored bit

Ignored bits must be preserved according to the ACPI spec.
Usually this means a read/modify/write when writing to the
register.  However, for status registers, writing a one means
clear the event.  Writing a zero means preserve the event (do not
clear.) This behavior is clarified in the ACPI 4.0 spec, and the
ACPICA code now simply always writes a zero to the ignored bit.

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>