cpufreq: Optimize cpufreq_show_cpus()

Instead of specially adding a space for each CPU, except the first one,
lets add space for each of them and remove it at the end.

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