pasemi_mac: 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 pasemi_mac, since
the device only exists as part of a big-endian system-on-chip, 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>