PM: wakeup: make device_wakeup_disable() return void

The device_wakeup_disable() call only returns an error if no dev exists,
but there's not much a user can do at that point.

Rather, make this function return void.

Signed-off-by: Dhruva Gole <d-gole@ti.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>