mac80211: use ERR_CAST()

No need for ERR_PTR(PTR_ERR()) since there's ERR_CAST, use it.
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/key.c

@@ -879,7 +879,7 @@ ieee80211_gtk_rekey_add(struct ieee80211_vif *vif,
 				  keyconf->keylen, keyconf->key,
 				  0, NULL);
 	if (IS_ERR(key))
-		return ERR_PTR(PTR_ERR(key));
+		return ERR_CAST(key);
 
 	if (sdata->u.mgd.mfp != IEEE80211_MFP_DISABLED)
 		key->conf.flags |= IEEE80211_KEY_FLAG_RX_MGMT;