LinuxKPI: 802.11: adjust vif->bss_conf.chanctx_conf to rcu accessors

Adjust the init and lkpi_sta_scan_to_auth() chanctx_conf accesses to
use rcu functions as needed.
linuxkpi_ieee80211_iterate_chan_contexts() which is
ieee80211_iter_chan_contexts_atomic() needs further work to get rid
of the vif locks by keeping all chanctx_conf on a list in lhw.
This will be done in a follow-up commit.

Sponsored by: The FreeBSD Foundation
PR: 280546
MFC after: 3 days
Tested by: Oleksandr Kryvulia (shuriku shurik.kiev.ua)
Tested by: Oleg Nauman (oleg.nauman gmail.com) [rtw88]
Differential Revision: https://reviews.freebsd.org/D49734