net/mlx5e: Init ethtool steering for representors

During transition to uplink representors the code responsible for
initializing ethtool steering functionality wasn't added to representor
init rx routine. This causes NULL pointer dereference during configuration
of network flow classification rule with ethtool (only possible to
reproduce with next commit in this series which registers necessary ethtool
callbacks).

Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>