LinuxKPI: 802.11 advertise full offload scanning based on hw_scan only

We disabled hw_scan for drivers not advertising SINGLE_SCAN_ON_ALL_BANDS.
Do not depend on this hw flag to set IEEE80211_FEXT_SCAN_OFFLOAD for
net80211 as otherwise scanning will never work.
Long-term we probably want to re-think how we do/integrate hw_scan
better in net80211.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days