atmel_serial: Fix tty_port breakage

The tty pointer has been moved into a tty_port field, so we need to use
->info->port.tty instead of just ->info->tty. Fixes these build errors:

David Brownell <david-b@pacbell.net> wrote:
> drivers/serial/atmel_serial.c: In function 'atmel_rx_from_ring':
> drivers/serial/atmel_serial.c:665: error: 'struct uart_info' has no member named 'tty'
> drivers/serial/atmel_serial.c: In function 'atmel_rx_from_dma':
> drivers/serial/atmel_serial.c:672: error: 'struct uart_info' has no member named 'tty'
> drivers/serial/atmel_serial.c: In function 'atmel_startup':
> drivers/serial/atmel_serial.c:797: error: 'struct uart_info' has no member named 'tty'
> make[2]: *** [drivers/serial/atmel_serial.o] Error 1

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>