[iwm] Use iwm_mvm_scan_stop_wait to properly abort scans.

• Add IWM_FLAG_SCAN_RUNNING to sc->sc_flags to track whether the firmware is currently running a scan, in order to decide wheter iwm_scan_end needs to abort a running scan.

• In iwm_scan_end, if the scan is still running, we now abort it, in order to keep the firmware scanning state in sync.

• Try to make things a bit simpler, by reacting on the IWM_SCAN_OFFLOAD_COMPLETE and IWM_SCAN_COMPLETE_UMAC notifications, instead of IWM_SCAN_ITERATION_COMPLETE and IWM_SCAN_ITERATION_COMPLETE_UMAC. This should be fine since we always only tell the firmware to do a single scan iteration anyway.

Obtained from: DragonflyBSD commit 1f249c981c4e89e7cde1836a75b61cac36dc7ac5