ACPI: glue: Change return type of two functions to void

Since the return values of acpi_device_notify() and
acpi_device_notify_remove() are discarded by their only caller,
change their return type to void.

No functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>