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

(cherry picked from commit c9ba91435a01a80632a5b68144d75b38241fc8be)