6lowpan: iphc: cleanup use of lowpan_fetch_skb

We introduced the lowpan_fetch_skb function in some previous patches for
6lowpan to have a generic fetch function. This patch drops the old
function and use the generic lowpan_fetch_skb one.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>