PM: wakeup: Update after recent wakeup source removal ordering change

PM: wakeup: Update after recent wakeup source removal ordering change

After a recent change, wakeup_source_activate() will warn that the given
wakeup source is "unregistered" after its timer has been shut down
in wakeup_source_remove() which may be somewhat confusing, so change
the warning message to say that the wakeup source is "unusable".

Accordingly, rename wakeup_source_not_registered() to
wakeup_source_not_usable() and update the comment in it
to also mention the removal of the wakeup source.

Also restore the comment in wakeup_source_remove() regarding the warning
in wakeup_source_activate() that may trigger after shutting down the
wakeup source timer.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/12788103.O9o76ZdvQC@rafael.j.wysocki