serial: sunsu: use container_of to resolve uart_sunsu_port from uart_port

Use container_of instead of casting first structure member.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>