mac80211: fix a possible leak of station stats

If sta_info_alloc fails after allocating the per CPU statistics,
they are not properly freed.

Fixes: c9c5962b

 ("mac80211: enable collecting station statistics per-CPU")
Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>