sandbox: mmc: Fix up MMC emulator for valgrind

At present running sandbox with valgrind produces some warnings due to the
MMC emulator not filling in all the expected fields. Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: use sizeof() instead of hardcoded 16]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/drivers/mmc/sandbox_mmc.c b/drivers/mmc/sandbox_mmc.c
index 2fa7d8c..899952d 100644
--- a/drivers/mmc/sandbox_mmc.c
+++ b/drivers/mmc/sandbox_mmc.c
@@ -27,6 +27,7 @@
 {
 	switch (cmd->cmdidx) {
 	case MMC_CMD_ALL_SEND_CID:
+		memset(cmd->response, '\0', sizeof(cmd->response));
 		break;
 	case SD_CMD_SEND_RELATIVE_ADDR:
 		cmd->response[0] = 0 << 16; /* mmc->rca */
@@ -43,11 +44,14 @@
 	case MMC_CMD_SEND_CSD:
 		cmd->response[0] = 0;
 		cmd->response[1] = 10 << 16;	/* 1 << block_len */
+		cmd->response[2] = 0;
+		cmd->response[3] = 0;
 		break;
 	case SD_CMD_SWITCH_FUNC: {
 		if (!data)
 			break;
 		u32 *resp = (u32 *)data->dest;
+		resp[3] = 0;
 		resp[7] = cpu_to_be32(SD_HIGHSPEED_BUSY);
 		if ((cmd->cmdarg & 0xF) == UHS_SDR12_BUS_SPEED)
 			resp[4] = (cmd->cmdarg & 0xF) << 24;