drivers/cpufreq/acpi-cpufreq.c: This fixes the following checkpatch warning

WARNING: Prefer using '"%s...", __func__' to using function's name, in a
string

Switch hardcoded function name with a reference to __func__ making the
code more maintainable

Signed-off-by: Mohan Kumar <mohankumar718@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>