cpufreq: Replace magic number

Setting the length of str_governor with a magic number could cause
overflow when max length increases, it is better to use the defined
macro in this case.

Signed-off-by: Bowen Yu <yubowen8@huawei.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://patch.msgid.link/20250519070908.930879-1-yubowen8@huawei.com


Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>