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

In the Linux kernel, the following vulnerability has been resolved:

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.