cpufreq: powernow-k8: Use related_cpus instead of cpus in driver.exit()

Since the 'cpus' field of policy structure will become empty in the
cpufreq core API, it is better to use 'related_cpus' in the exit()
callback of driver.

Fixes: c3274763 ("cpufreq: powernow-k8: Initialize per-cpu data-structures properly")
Signed-off-by: Liao Chang <liaochang1@huawei.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>