PM: sleep: stats: Use unsigned int for success and failure counters

Change the type of the "success" and "fail" fields in struct
suspend_stats to unsigned int, because they cannot be negative.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>