cpufreq: Use list_is_last() to check last entry of the policy list

Currently next_policy() explicitly checks if a policy is the last
policy in the cpufreq_policy_list. Use the standard list_is_last
primitive instead.

Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>