mac80211: fix statistics leak if dev_alloc_name() fails

In the case that dev_alloc_name() fails, e.g. because the name was
given by the user and already exists, we need to clean up properly
and free the per-CPU statistics. Fix that.

Cc: stable@vger.kernel.org
Fixes: 5a490510

 ("mac80211: use per-CPU TX/RX statistics")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>