cpufreq: brcmstb-avs-cpufreq: add check for cpufreq_cpu_get's return value

cpufreq_cpu_get may return NULL. To avoid NULL-dereference check it
and return 0 in case of error.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: de322e08 ("cpufreq: brcmstb-avs-cpufreq: AVS CPUfreq driver for Broadcom STB SoCs")
Signed-off-by: Anastasia Belova <abelova@astralinux.ru>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>