cadence_uart: Fix buffer overflow

Report from smatch:
hw/cadence_uart.c:413 uart_read(13) error: buffer overflow 's->r' 18 <= 18

This fixes read access to s->r[R_MAX] which is behind the limits of s->r.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>