cpufreq: apple-soc: Fix null-ptr-deref in apple_soc_cpufreq_get_rate()

cpufreq_cpu_get_raw() can return NULL when the target CPU is not present
in the policy->cpus mask. apple_soc_cpufreq_get_rate() does not check
for this case, which results in a NULL pointer dereference.

Fixes: 6286bbb4 ("cpufreq: apple-soc: Add new driver to control Apple SoC CPU P-states")
Signed-off-by: Henry Martin <bsdhenrymartin@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>