suspend: use WARN not WARN_ON to print the message

By using WARN(), kerneloops.org can collect which component is causing
the delay and make statistics about that. suspend_test_finish() is
currently the number 2 item but unless we can collect who's causing
it we're not going to be able to fix the hot topic ones..

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>