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>