ethtool: Fix check in ethtool_rx_flow_rule_create

Fix check in ethtool_rx_flow_rule_create

Fixes: eca4205f ("ethtool: add ethtool_rx_flow_spec to flow_rule structure translator")
Signed-off-by: Gaurav Singh <gaurav1086@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>