bluetooth: 6lowpan: use lowpan dispatch helpers

This patch adds a check if the dataroom of skb contains a dispatch value
by checking if skb->len != 0. This patch also change the dispatch
evaluation by the recently introduced helpers for checking the common
6LoWPAN dispatch values for IPv6 and IPHC header.

There was also a forgotten else branch which should drop the packet if
no matching dispatch is available.

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>