nl80211: Fix external_auth check for offloaded authentication

Unfortunately removal of the ext_feature flag in the last revision of
the patch ended up negating the comparison and prevented the command
from being processed (either nl80211_external_auth() or
rdev_external_auth() returns -EOPNOTSUPP). Fix this by adding back the
lost '!'.

Fixes: 40cbfa90 ("cfg80211/nl80211: Optional authentication offload to userspace")
Signed-off-by: Srinivas Dasari <dasaris@codeaurora.org>
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/nl80211.c

@@ -12484,7 +12484,7 @@ static int nl80211_external_auth(struct sk_buff *skb, struct genl_info *info)
 	struct net_device *dev = info->user_ptr[1];
 	struct cfg80211_external_auth_params params;
 
-	if (rdev->ops->external_auth)
+	if (!rdev->ops->external_auth)
 		return -EOPNOTSUPP;
 
 	if (!info->attrs[NL80211_ATTR_SSID])