6lowpan: fix type of datagram size parameter

Datagram size value is u16 because we convert it to host byte order
and we need to read it. Only the tag value belongs to __be16 type.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc net/ieee802154/reassembly.c
Simple merge