bnx2/bnx2x: Unsupported Ethtool operations should return -EINVAL.

Some cards don't support changing vlan offloading settings.  Make
Ethtool set_flags return -EINVAL in those cases.

Reported-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>