PM / wakeup: only recommend "call"ing device_init_wakeup() once

I'll admit admit it: I've written bad driver code that tries to
configure a device's wake IRQ without having called device_init_wakeup()
first. But do you really have to ask ask me twice?

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>