PM: Remove WARN_ON from device_pm_add

Fix message in device_pm_add() saying that the device will not be
added to dpm_list, although in fact the device is going to be added
to the list regardless of the ordering violation.

Remove the WARN_ON(true) triggered in that situation, because it is
hit by USB very often and spams the users' logs.

This patch fixes bug #11263
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>