mac80211: add missing channel context release

commit 2a3ba63c235fdcd37f6451bdf4a0c7865a3930cf upstream.

IBSS needs to release the channel context when leaving
but I evidently missed that. Fix it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>