__ethtool_set_flags() was not taking into account features set but not
user-toggleable.

Since GFLAGS returns masked dev->features, EINVAL is returned when
passed flags differ to it, and not to wanted_features.
Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>