ixgbe: fix ethtool disable csum for ipv6

ethtool was not disabling the correct netif flags when setting
checksum disable.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>