aspeed/smc: fix default read value

0xFFFFFFFF should be returned for non implemented registers.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Message-id: 20190124140519.13838-2-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/ssi/aspeed_smc.c

@@ -670,7 +670,7 @@ static uint64_t aspeed_smc_read(void *opaque, hwaddr addr, unsigned int size)
     } else {
         qemu_log_mask(LOG_UNIMP, "%s: not implemented: 0x%" HWADDR_PRIx "\n",
                       __func__, addr);
-        return 0;
+        return -1;
     }
 }