cpufreq/amd-pstate: handle missing policy in dynamic EPP callbacks

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

cpufreq/amd-pstate: handle missing policy in dynamic EPP callbacks

cpufreq_cpu_get() returns NULL when no cpufreq policy is associated with
the requested CPU, for example because the CPU is offline or the policy
has already been torn down. Both amd_pstate_power_supply_notifier() and
amd_pstate_profile_set() acquire a policy via cpufreq_cpu_get() and then
pass that pointer to amd_pstate_get_balanced_epp() and
amd_pstate_set_epp(), which dereference it unconditionally. A racing
CPU hotplug or driver teardown can therefore lead to a NULL pointer
dereference on either of these dynamic EPP paths.

The third cpufreq_cpu_get() caller in this file, amd_pstate_verify(),
already handles the NULL case. Bring the two new callers in line with
that pattern: return NOTIFY_OK from the power-supply notifier (matching
the other "nothing to do" exits) and -ENODEV from amd_pstate_profile_set()
(the usual cpufreq error for a missing CPU policy).

Found by code inspection; not tested on hardware.