Some cards don't support changing vlan offloading settings.  Make
Ethtool set_flags return -EINVAL in those cases.
Reported-by: NBen Hutchings <bhutchings@solarflare.com>
Signed-off-by: NJesse Gross <jesse@nicira.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>