net80211: mark argument to scan_curchan_task() __unused

Mostly as documentation mark an unused argument to scan_curchan_task()
as __unused. We may possibly want to check all callers in the future
and see if the argument was supposed to be useful or should be entirely
removed.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 778da03b1271f52051620ec9b053524727961c9e)