ieee802154: 6lowpan: don't skip first dsn while fragmentation

This patch fixes the data frame sequence numer (dsn) while 6lowpan
fragmentation for frag1. Currently we create one 802.15.4 header at
first, then check if it's match into one frame and at the end construct
many fragments and calling wpan_dev_hard_header for each of them,
inclusive for the first fragment. This will make the first generated
header to garbage, instead we copying this header for frag1 instead of
generate a new one which skips one dsn.

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