cpufreq/amd-pstate: Add missing NULL ptr check in amd_pstate_update

Check if policy is NULL before dereferencing it in amd_pstate_update.

Fixes: e8f555da ("cpufreq/amd-pstate: fix setting policy current frequency value")
Signed-off-by: Dhananjay Ugwekar <dhananjay.ugwekar@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Link: https://lore.kernel.org/r/20250205112523.201101-11-dhananjay.ugwekar@amd.com


Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>