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

hw/cadence_uart.c

@@ -404,7 +404,7 @@ static uint64_t uart_read(void *opaque, target_phys_addr_t offset,
     uint32_t c = 0;
 
     offset >>= 2;
-    if (offset > R_MAX) {
+    if (offset >= R_MAX) {
         return 0;
     } else if (offset == R_TX_RX) {
         uart_read_rx_fifo(s, &c);