cpufreq: Use WARN_ON_ONCE() for invalid relation

The relation can't be invalid here, so if it turns out to be invalid,
just WARN_ON_ONCE() and return 0.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>