mac802154: check on len instead mac_len

This patch change the length check to len instead of mac_len for
checking if the frame control field is available to dereference.
We need to change it because I saw issues with af_packet raw sockets
and the mrf24j40 which calls this functionality. The raw socket
functionality doesn't set the mac_len but resets the skb_mac_header to
skb->data which is still correct. The issue occur at mrf24j40 only,
because the driver need to evaluate the fc fields.
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

include/net/mac802154.h

@@ -248,7 +248,7 @@ struct ieee802154_ops {
 static inline __le16 ieee802154_get_fc_from_skb(const struct sk_buff *skb)
 {
 	/* return some invalid fc on failure */
-	if (unlikely(skb->mac_len < 2)) {
+	if (unlikely(skb->len < 2)) {
 		WARN_ON(1);
 		return cpu_to_le16(0);
 	}