mac80211: add missing channel context release

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

Cc: stable@vger.kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/ibss.c

@@ -1138,6 +1138,7 @@ int ieee80211_ibss_leave(struct ieee80211_sub_if_data *sdata)
 	clear_bit(SDATA_STATE_OFFCHANNEL_BEACON_STOPPED, &sdata->state);
 	ieee80211_bss_info_change_notify(sdata, BSS_CHANGED_BEACON_ENABLED |
 						BSS_CHANGED_IBSS);
+	ieee80211_vif_release_channel(sdata);
 	synchronize_rcu();
 	kfree(presp);