net: contiki: Do not discard non-fragmented packets

If the fragmentation header is missing, then do not discard
the message but pass it to upper layers as is. The fragmentation
header might be missing if the packet fits into 128 bytes.

Change-Id: I39d29e7ad80be5531262a0ac413d8137da0c64f5
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>