cpufreq: optimize policy_is_shared()

The switch to cpumask_nth() over cpumask_weight(), as it may return
earlier - as soon as the function counts the required number of CPUs.

Signed-off-by: Yury Norov <ynorov@nvidia.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>
Link: https://patch.msgid.link/20260314192544.605914-1-ynorov@nvidia.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>