ACPI: change init_acpi_device_notify() to return void

This patch changes the type of the return value of the init_acpi_device_notify()
method to be void, as this method never fails and its return value is never
used.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>