cpufreq: conservative: Rename get_freq_target() to get_freq_step()

What's returned from this function is the delta by which the frequency
must be increased or decreased and not the final frequency that should
be selected.

Name it properly to match its purpose. Also update the variables used to
store that value.

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