LinuxKPI: 802.11: improve scan handling

Under certain circumstances a hw_scan may be downgraded to a software
scan. Handle these situations better and make sure we free resources
in all cases once. [1]

Also leave a note about scanning all bands (or we would have to switch
bands manually).

In both cases hardware doing and driver saying seem not entirely
consistent for all and all firmware.

Sponsored by: The FreeBSD Foundation
MFC after: 10 days
Reported by: imp [1]