cpufreq: sparc: change kzalloc to kcalloc

Refactor to use kcalloc instead of kzalloc when multiplying
allocation size by count. This refactor prevents unintentional
memory overflows. Discovered by checkpatch.pl.

Signed-off-by: Ethan Carter Edwards <ethan@ethancedwards.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>