mac80211: release the channel in error path in start_ap

commit 0297ea17bf7879fb5846fafd1be4c0471e72848d upstream.

When the driver cannot start the AP or when the assignement
of the beacon goes wrong, we need to unassign the vif.

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