LiunxKPI: 802.11: move ieee80211_chanctx_conf into lkpi private struct

Factor out ieee80211_chanctx_conf into struct lkpi_chanctx in order to
keep local state as well. In first instance that is added_to_drv
only. For now we stay single-chanctx only but this paves the path
to make it a list.
Use the new information to implement ieee80211_iter_chan_contexts_atomic().

Sponsored by: The FreeBSD Foundation
MFC after: 3 days