cpufreq: CPPC: Do not return a value from populate_efficiency_class()

The return value of populate_efficiency_class() is never needed and the
result of it doesn't affect the initialization of cppc_cpufreq.

It makes more sense to change it into a void function.

Signed-off-by: Lifeng Zheng <zhenglifeng1@huawei.com>
Link: https://patch.msgid.link/20250526113057.3086513-3-zhenglifeng1@huawei.com


[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>