xen: netfront: ethtool stats fields should be unsigned long

Fixup the rx_gso_checksum_fixup field added in e0ce4af9 to be
unsigned long as suggested by Ben Hutchings in
<1298919198.2569.14.camel@bwh-desktop>

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>