cpufreq: mvebu: Free opp if registering failed

Since the introduction of this driver, the functions to remove the opp
were added. So stop claiming we can't remove opp and use one of them in
case of failure.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>