LinuxKPI: 802.11: rename chanctx_conf for consistency

We used to call the struct ieee80211_chanctx_conf variable "conf"
in some places but that becomes confusing with bss_conf and other
"conf" bits. On the vif it is already called chanctx_conf thus also
rename it on the internal struct lkpi_chanctx and for our variables
in the implementation.

This should not have any external visibility.
No functional changes intended.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D45185