net/mlx5e: Save the represntor netdevice as part of the representor

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: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>