mac80211: silently accept deletion of non-existant key

Otherwise, 'iwconfig wlan0 key off' with no key set results in:

	Error for wireless request "Set Encode" (8B2A) :
	    SET failed on device wlan0 ; No such file or directory.
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/ieee80211_ioctl.c

@@ -55,9 +55,6 @@ static int ieee80211_set_encryption(struct net_device *dev, u8 *sta_addr,
 			key = sta->key;
 		}
 
-		if (!key)
-			return -ENOENT;
-
 		ieee80211_key_free(key);
 		return 0;
 	} else {