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