cpufreq: intel_pstate: Shorten a couple of long names

The names of the INTEL_PSTATE_DEFAULT_SAMPLING_INTERVAL symbol and
the get_target_pstate_use_cpu_load() function don't need to be so
long any more, so make them shorter.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>