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

commit 5493f971 upstream.

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.

Signed-off-by: Anastasia Belova <abelova@astralinux.ru>
Reviewed-by: Perry Yuan <perry.yuan@amd.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
[ Raj: on 6.6, there don't have function amd_pstate_update_limits()
  so applied the NULL checking in amd_pstate_adjust_perf() only ]
Signed-off-by: Rajani Kantha <rajanikantha@engineer.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>