ieee802154: cleanup WARN_ON for fc fetch

This patch cleanups the WARN_ON which occurs when the sk buffer has
insufficient buffer space by moving the WARN_ON into if condition.
Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

include/net/mac802154.h

@@ -250,11 +250,10 @@ static inline __le16 ieee802154_get_fc_from_skb(const struct sk_buff *skb)
 	__le16 fc;
 
 	/* check if we can fc at skb_mac_header of sk buffer */
-	if (unlikely(!skb_mac_header_was_set(skb) ||
-		     (skb_tail_pointer(skb) - skb_mac_header(skb)) < 2)) {
-		WARN_ON(1);
+	if (WARN_ON(!skb_mac_header_was_set(skb) ||
+		    (skb_tail_pointer(skb) -
+		     skb_mac_header(skb)) < IEEE802154_FC_LEN))
 		return cpu_to_le16(0);
-	}
 
 	memcpy(&fc, skb_mac_header(skb), IEEE802154_FC_LEN);
 	return fc;