CPU/hotplug: Clarify description of __cpuhp_setup_state() return value

When ivoked with CPUHP_AP_ONLINE_DYN state __cpuhp_setup_state()
is expected to return positive value which is the hotplug state that
the routine assigns.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>