nl80211: add request id in scheduled scan event messages

For multi-scheduled scan support in subsequent patch a request id
will be added. This patch add this request id to the scheduled
scan event messages. For now the request id will always be zero.
With multi-scheduled scan its value will inform user-space to which
scan the event relates.

Reviewed-by: Hante Meuleman <hante.meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>