ACPICA: Invalidate DdbHandle after table unload

Mark the DdbHandle as invalid after the table it refers to is
unloaded.  This is needed because the handle itself may not be
deleted after the table unload, depending on whether it has been
stored in a named object by the caller.

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