cpufreq: Break out early when frequency equals target_freq

Many drivers keep frequencies in frequency table in ascending
or descending order. When governor tries to change to policy->min
or policy->max respectively then the cpufreq_frequency_table_target
could return on first iteration. This will save some iteration cycles.

So, break out early when a frequency in cpufreq_frequency_table
equals to target one.

Testing this during kernel compilation using ondemand governor
with a frequency table in ascending order, the
cpufreq_frequency_table_target returned early on the first
iteration at about 30% of times called.

Signed-off-by: Stratos Karafotis <stratosk@semaphore.gr>