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>

include/linux/cpufreq.h

@@ -956,8 +956,8 @@ static inline int cpufreq_frequency_table_target(struct cpufreq_policy *policy,
 	case CPUFREQ_RELATION_C:
 		return cpufreq_table_find_index_c(policy, target_freq);
 	default:
-		pr_err("%s: Invalid relation: %d\n", __func__, relation);
-		return -EINVAL;
+		WARN_ON_ONCE(1);
+		return 0;
 	}
 }