mac80211: use chanctx reservation for STA CSA

Channel switch finalization is now 2-step. First
step is when driver calls chswitch_done(), the
other is when reservation is actually finalized
(which be defered for in-place reservation).

It is now safe to call ieee80211_chswitch_done()
more than once.

Also remove the ieee80211_vif_change_channel()
because it is no longer used.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>