msm: timer: use WARN instead of WARN_ON

Previously a format string was incorrectly passed to WARN_ON(), causing the
WARN to correctly always trigger, but not print the string in question.
Fix this by switching to WARN().

Change-Id: Iaa41dea7a1097f2b629dba0bf8a75b8b6072200c
Signed-off-by: Jeff Ohlstein <johlstei@codeaurora.org>