cpufreq: st: add missing \n to end of dev_err message

Trival fix, dev_err message is missing a \n, so add it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>