ACPI / notify: Do not block unknown type notifications in root handler

Commit 1a699476 "ACPI / hotplug / PCI: Hotplug notifications from
acpi_bus_notify()" changed the root notify handler, acpi_bus_notify(),
to block unknown type norifications, but it overlooked the fact that
they might be propagated to drivers via the ->notify() callback.

Fix the problem by allowing drivers to receive unknown type
notifications via ->notify() as before.

Fixes: 1a699476

 (ACPI / hotplug / PCI: Hotplug notifications from acpi_bus_notify())
Reported-and-tested-by: Mantas Mikulėnas <grawity@gmail.com>
Reported-and-tested-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>