ieee802154: reassembly: fix tag byteorder

This patch fix byte order handling in reassembly code of 802.15.4
6LoWPAN fragmentation handling.

net/ieee802154/reassembly.c:58:43: warning: restricted __be16 degrades to integer

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