cfg80211: avoid mem leak on driver hint set

In the already-set and intersect case of a driver-hint, the previous
wiphy regdomain was not freed before being reset with a copy of the
cfg80211 regdomain.

Cc: stable@vger.kernel.org
Signed-off-by: Arik Nemtsov <arikx.nemtsov@intel.com>
Acked-by: Luis R. Rodriguez <mcgrof@suse.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>