cppc_cpufreq: Fix possible null pointer dereference

cppc_cpufreq_get_rate() and hisi_cppc_cpufreq_get_rate() can be called from
different places with various parameters. So cpufreq_cpu_get() can return
null as 'policy' in some circumstances.
Fix this bug by adding null return check.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: a28b2bfc ("cppc_cpufreq: replace per-cpu data array with a list")
Signed-off-by: Aleksandr Mishin <amishin@t-argos.ru>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>