cpufreq: brcmstb-avs-cpufreq: make symbol brcm_avs_cpufreq_attr static

Fixes the following sparse warning:

drivers/cpufreq/brcmstb-avs-cpufreq.c:982:18: warning:
 symbol 'brcm_avs_cpufreq_attr' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Markus Mayer <mmayer@broadcom.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>