cpufreq: ARM big LITTLE DT: Return correct transition latency

By mistake we are returning zero for successful call to
dt_get_transition_latency(), whereas we should return
transition_latency.  Fix that.

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