ieee802154: 6lowpan: fix byteorder for frag tag

This patch fix byteorder issues with fragment tag of generation 802.15.4
6LoWPAN fragment header.

net/ieee802154/6lowpan_rtnl.c:278:54: warning restricted __be16 degrades to integer
net/ieee802154/6lowpan_rtnl.c:278:18: warning: incorrect type in assignment (different base types)
net/ieee802154/6lowpan_rtnl.c:278:18:    expected restricted __be16 [usertype] frag_tag
net/ieee802154/6lowpan_rtnl.c:278:18:    got unsigned short

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