cpufreq: Avoid using global variable total_cpus

The change is to compile on kernels where cpufreq stats are compiled as
a module (CONFIG_CPU_FREQ_STAT=m), because total_cpus is not exported for
module use.

Reported-By: Emilio López <elopez93@gmail.com>
Signed-off-by: Ruchi Kandoi <kandoiruchi@google.com>
Change-Id: I4f3c74f0fac5e8d9449655b26bf3b407b0fe4290