cfg80211: fix possible memory leak in cfg80211_iter_combinations()

'limits' is malloced in cfg80211_iter_combinations() and should be freed
before leaving from the error handling cases, otherwise it will cause
memory leak.

Fixes: 0c317a02 ("cfg80211: support virtual interfaces with different beacon intervals")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>