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

commit 503bded92da283b2f31d87e054c4c6d30c3c2340 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>