cpufreq: No need to verify cpufreq_driver in show_scaling_cur_freq()

"cpufreq_driver" is guaranteed to be valid here, no need to check it
here.

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