Replace the representor private data to a net_device pointer holding the
representor netdevice, instead of void pointer holding mlx5e_priv.

It will be used by a new eswitch service function, returning the uplink representor
netdevice.
Signed-off-by: NHadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>