net80211: ieee80211_swscan_bg_scan() track return variable under lock

As the comment says it probably does not matter but use a local
variable to track state under lock so we can return the last known
good state of what we thought we were operating under after unlocking.

Likely no functional changes.

Sponsored by: The FreeBSD Foundation
MFC atfer: 3 days
Reviewed by: enweiwu, adrian
Differential Revision: https://reviews.freebsd.org/D38660