cpufreq: CPPC: Reuse caps variable in few routines

The 'caps' variable has been defined in cppc_cpufreq_khz_to_perf() and
cppc_cpufreq_perf_to_khz() routines, so there is no need to get
'highest_perf' value through 'cpu->caps.highest_perf', we can use
'caps->highest_perf' instead.

Signed-off-by: Xin Hao <xhao@linux.alibaba.com>
[ Viresh: Updated commit log ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>