cpufreq: Drop 'freq_table' argument of __target_index()

It is already present as part of the policy and so no need to pass it
from the caller. Also, 'freq_table' is guaranteed to be valid in this
function and so no need to check it.

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