net: ieee802154: remove explicit set skb->sk

Explicit set skb->sk is needless, sock_alloc_send_skb is already set it.

Signed-off-by: Lin Zhang <xiaolou4617@gmail.com>
Acked-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>