ACPI: don't pass handle for fixed hardware notifications

Fixed hardware devices have no handles, so just pass an explicit
NULL rather than something that looks like it might be meaningful.
acpi_device_notify() doesn't need the handle anyway; the only
reason it takes it as an argument is because the acpi_notify_handler
typedef requires it.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>