cpufreq: Drop unnecessary check from cpufreq_policy_alloc()

Since cpufreq_policy_alloc() doesn't use its dev variable for
anything useful, drop that variable from there along with the
NULL check against it.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>