cpufreq: arm_scmi: Fix error path when allocation failed

Stop the initialization when cpumask allocation failed and return an
error.

Fixes: 80a064db ("scmi-cpufreq: Get opp_shared_cpus from opp-v2 for EM")
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>