cpufreq: CPPC: Remove forward declaration of cppc_cpufreq_register_em()

cpufreq: CPPC: Remove forward declaration of cppc_cpufreq_register_em()

cppc_cpufreq_register_em() is only used in populate_efficiency_class(). A
forward declaration of it is not necessary.

Move cppc_cpufreq_register_em() in front of populate_efficiency_class()
and remove the forward declaration of cppc_cpufreq_register_em().

No functional change.

Signed-off-by: Lifeng Zheng <zhenglifeng1@huawei.com>
Link: https://patch.msgid.link/20250526113057.3086513-4-zhenglifeng1@huawei.com
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>