cpufreq: Drop cpufreq_disabled() check from cpufreq_cpu_{get|put}()

When cpufreq is disabled, the per-cpu variable would have been set to
NULL. Remove this unnecessary check.

[ Changelog from Saravana Kannan. ]

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