cpufreq: mediatek: Cleanup variables and error handling in mtk_cpu_dvfs_info_init()

- Remove several unnecessary varaibles in mtk_cpu_dvfs_info_init().
- Unify error message format and use dev_err_probe() if possible.

Signed-off-by: Jia-Wei Chang <jia-wei.chang@mediatek.com>
Signed-off-by: Rex-BC Chen <rex-bc.chen@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>