cpufreq: mediatek: add missing platform_driver_unregister() on error in mtk_cpufreq_driver_init

Add the missing platform_driver_unregister() before return from
mtk_cpufreq_driver_init in the error handling case when failed
to register mtk-cpufreq platform device

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>