cpufreq: amd-pstate: add check for cpufreq_cpu_get's return value

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

cpufreq: amd-pstate: add check for cpufreq_cpu_get's return value

cpufreq_cpu_get may return NULL. To avoid NULL-dereference check it

and return in case of error.

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