cpufreq: intel_pstate: Clean up aperf_mperf_shift description

The kerneldoc description of the aperf_mperf_shift field in
struct global_params is unclear and there is a typo in it, so
simplify it and clean it up.

Reported-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lee Jones <lee.jones@linaro.org>