cpufreq: scpi: Fix null-ptr-deref in scpi_cpufreq_get_rate()

cpufreq_cpu_get_raw() can return NULL when the target CPU is not present
in the policy->cpus mask. scpi_cpufreq_get_rate() does not check for
this case, which results in a NULL pointer dereference.

Fixes: 343a8d17 ("cpufreq: scpi: remove arm_big_little dependency")
Signed-off-by: Henry Martin <bsdhenrymartin@gmail.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>