wifi: mac80211: tx: simplify chanctx_conf handling

In ieee80211_build_hdr() we do the same thing for all
interface types except for AP_VLAN, but we can simplify
the code by pulling the common thing in front of the
switch and overriding it for AP_VLAN. This will also
simplify the code for MLD here later.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>