atmel_serial: use container_of instead of direct cast

As pointed out by David Brownell, we really ought to be using container_of
when converting from "struct uart_port *" to "struct atmel_uart_port *".

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Cc: Andrew Victor <linux@maxim.org.za>
Tested-by: Marc Pignat <marc.pignat@hevs.ch>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>