cpufreq: unicore2: Staticize local symbol

This local symbol is used only in this file.
Fix the following sparse warnings:

drivers/cpufreq/unicore2-cpufreq.c:27:5: warning: symbol 'ucv2_verify_speed' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>