sm501: Add missing break to case

Noticed by Coverity, forgotten in 5690d9ecReported-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NBALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

hw/display/sm501.c

@@ -797,6 +797,7 @@ static uint64_t sm501_system_config_read(void *opaque, hwaddr addr,
         break;
     case SM501_COMMAND_LIST_STATUS:
         ret = 0x00180002; /* FIFOs are empty, everything idle */
+        break;
     case SM501_IRQ_MASK:
         ret = s->irq_mask;
         break;