mlxsw: Call free_netdev when removing port

When removing a port's netdevice we should also free the memory
allocated by alloc_etherdev(). Do this by calling free_netdev() at the
end of the teardown sequence.

Reported-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>