cpufreq: mediatek: Convert pr_warn() to pr_debug()

With multi-platform build images, this shows a message on non mediatek
platforms, which is unnecessary. Convert pr_warn() to pr_debug() here.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>