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
Reported by: imp [1]

(cherry picked from commit 3206587a20de9810b549ba8fd0032f7ab884bbec)