cpufreq/amd-pstate: Add the missing cpufreq_cpu_put()

Fix the reference counting of cpufreq_policy object in amd_pstate_update()
function by adding the missing cpufreq_cpu_put().

Fixes: e8f555da ("cpufreq/amd-pstate: fix setting policy current frequency value")
Signed-off-by: Dhananjay Ugwekar <Dhananjay.Ugwekar@amd.com>
Reviewed-by: Perry Yuan <perry.yuan@amd.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>