cpufreq: mediatek-hw: Fix wrong return value in mtk_cpufreq_get_cpu_power()

mtk_cpufreq_get_cpu_power() return 0 if the policy is NULL. Then in
em_create_perf_table(), the later zero check for power is not invalid
as power is uninitialized. As Lukasz suggested, it must return -EINVAL when
the 'policy' is not found. So return -EINVAL to fix it.

Cc: stable@vger.kernel.org
Fixes: 4855e26b ("cpufreq: mediatek-hw: Add support for CPUFREQ HW")
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Suggested-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>