cpufreq: Move ->get callback check outside of __cpufreq_get()

Currenly, __cpufreq_get() called by show_cpuinfo_cur_freq() will check
->get callback. That is needless since cpuinfo_cur_freq attribute will
not be created if ->get is not set. So let's drop it in __cpufreq_get().
Also keep this check in cpufreq_get().

Signed-off-by: Yue Hu <huyue2@yulong.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>