mac80211: release the channel in error path in start_ap

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

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