ieee80211_vif_change_channel() locks chanctx_mtx.  When implementing
channel reservation for CS, we will need to call the function to
change channel when the lock is already held, so split the part that
requires the lock out and leave the locking in the original function.
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>