net80211: make ieee80211_scan_dump_channels private

ieee80211_scan_dump_channels() is only used locally and only when
IEEE80211_DEBUG is compiled. Stop exporting it, make it file local
and hide under the #ifdef to reduce the footprint for production
kernels a tiny bit.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D38833