rocker: free netdevice during netdevice removal

When removing a port's netdevice in 'rocker_remove_ports', we should
also free the allocated 'net_device' structure. Do that by calling
'free_netdev' after unregistering it.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Fixes: 4b8ac966

 ("rocker: introduce rocker switch driver")
Acked-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>