netdev: Make ethtool_ops::set_flags() return -EINVAL for unsupported flags

The documented error code for attempts to set unsupported flags (or
to clear flags that cannot be disabled) is EINVAL, not EOPNOTSUPP.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Acked-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>