6lowpan: remove lowpan_fetch_skb_u8

This patch removes the lowpan_fetch_skb_u8 function for getting the iphc
bytes. Instead we using the generic which has a len parameter to tell
the amount of bytes to fetch.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>