cfg80211: Fix memory leak

When a driver requests a specific regulatory domain after cfg80211 already
has one, a struct ieee80211_regdomain is leaked.

Reported-by: Larry Finger <Larry.Finger@lwfinger.net>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>