LinuxKPI: 802.11: adjust locking around lkpi_remove_chanctx()

With the adjusted locking as a first step and the single code path
into lkpi_80211_mo_unassign_vif_chanctx() we also move the "setting
chnactx to NULL" into the caller to simplify the code.

Before we can adjust the other places touching vif->bss_conf.chanctx
we need to adjust the overall locking in LinuxKPI 802.11.

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 231168c7e77777f69a323bcd80a7956527d34548)