net: fix ETHTOOL_SFEATURES compatibility with old ethtool_ops.set_flags

Current code squashes flags to bool - this makes set_flags fail whenever
some ETH_FLAG_* equivalent features are set. Fix this.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>