xen: netfront: fix xennet_get_ethtool_stats()

commit e9a799ea (xen: netfront: ethtool stats fields should be
unsigned long) made rx_gso_checksum_fixup an unsigned long.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>