ethtool: Add hw-switch-offload to netdev_features_strings.

commit aafb3e98 (netdev: introduce new NETIF_F_HW_SWITCH_OFFLOAD feature
flag for switch device offloads) add a new feature without adding it to
netdev_features_strings array; this patch fixes this.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>