LinuxKPI: 802.11: improve hw_scan fallback to sw_scan

Extending what was started in d3ef7fb459ff924911e5276db0c04b13cd8074d9,
when a driver signals that hw_scan is not possible and expects a sw_scan
to be preformed we triggered a sw_scan towards the driver but did not
let net80211 know.
Cancel the initial scan towards net80211. If we we defer to sw_scan
then clear IEEE80211_FEXT_SCAN_OFFLOAD so net80211 will send probe
requests, and actively start a new scan with net80211.
This may have to be further refined in the future but seems to work
for the moment.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days