ehea: Access iph->tot_len with correct endianness

iph->tot_len is stored in network byte order, so access it using
ntohs().  This doesn't have any real world impact on ehea, since ehea
only exists for big-endian platfroms (at the moment at least) but fixing
this gets rid of a sparse warning and avoids having a bad example in the
tree.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>