cpufreq: Return error if ->get() failed in cpufreq_update_policy()

cpufreq_update_policy() calls cpufreq_driver->get() to get current
frequency of a CPU and it is not supposed to fail or return zero.
Return error in case that happens.

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