LinuxKPI: 802.11: add a lchanctx list to lhw

While we are currently only supporting a single VAP (vif) it is
less of a trouble but in order to get locking and rcu accesses
[from drivers] more right add a list for all chanctx_conf we have
for one hw. Use that list in the iterator function to avoid
having to lock the vif but not protecting the chanctx list
(against a parallel removal) due to different locking.

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

(cherry picked from commit a8a47a41775b0320606f90b3ac2048bc23494615)