cpufreq: intel_pstate: Fix plain int as pointer warning from sparse

Fix sparse warning: Using plain integer as NULL pointer.

Replace assignment of 0 to pointers with NULL assignment.

Signed-off-by: Jamal Shareef <jamal.k.shareef@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>