mac802154: fix mac header length check

I got report about that sometimes the WARN_ON occurs there which should
never happen. I came to the conclusion that the mac header is there but
inside the headroom of skb. The skb->len information doesn't contain the
information about the headroom length and skb->len is lesser than two.

We check now if the skb_mac_header pointer is set and the room between
mac header pointer and tail pointer.

Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>