mac80211: explicitly copy channels to VLANs where needed

Currently the code assigns channel contexts to VLANs
(for use by the TX/RX code) when the AP master gets
its channel context assigned. This works fine, but
in the upcoming radar detection work the VLANs don't
require a channel context (during radar detection)
and assigning one to them anyway causes issues with
locking and also inconsistencies -- a VLAN interface
that is added before radar detection would get the
channel context, while one added during it wouldn't.

Fix these issues moving the channel context copying
to a new explicit operation that will not be used
in the radar detection code.

Acked-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>