tty: atmel_serial_probe(): index of atmel_ports[] fix

Index of atmel_ports[ATMEL_MAX_UART] should be smaller
than ATMEL_MAX_UART.

Signed-off-by: Pawel Wieczorkiewicz <wpawel@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>