ieee802154: 6lowpan: cleanup pull of iphc bytes

This patch cleanups the pull of the iphc bytes. We don't need to check
if the skb->len contains two bytes, this will be checked by
lowpan_fetch_skb_u8.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>