cfg80211: remove obsolete comment for .sched_scan_stop() callback

The kerneldoc comment for .sched_scan_stop() callback describes a
driver_initiated flag, but the interface does not hold such a flag.

Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>