thermal: devfreq_cooling: Replace dev_warn with dev_err

There isn't much the user can do on seeing this warning, as the hardware
is actually okay. dev_err suits much better here.
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>

drivers/thermal/devfreq_cooling.c

@@ -193,7 +193,7 @@ get_static_power(struct devfreq_cooling_device *dfc, unsigned long freq)
 	dev_pm_opp_put(opp);
 
 	if (voltage == 0) {
-		dev_warn_ratelimited(dev,
+		dev_err_ratelimited(dev,
 				    "Failed to get voltage for frequency %lu: %ld\n",
 				    freq, IS_ERR(opp) ? PTR_ERR(opp) : 0);
 		return 0;