Per sections 3.6.1 (SD Bus Protocol), 4.3.4 "Data Write"
and 7.3.2 (Responses):

  In the CMD line the Most Significant Bit is transmitted first.

Use the stl_be_p() helper to store the value in big-endian.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Cédric Le Goater <clg@redhat.com>
---
RFC because I'm surprised this has been unnoticed for 17 years
(commit a1bb27b1e9 "initial SD card emulation", April 2007).

Cc: Peter Maydell <peter.maydell@linaro.org>
---
 hw/sd/sd.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/hw/sd/sd.c b/hw/sd/sd.c
index 4e09640852..1f37d9c93a 100644
--- a/hw/sd/sd.c
+++ b/hw/sd/sd.c
@@ -1668,8 +1668,7 @@ static sd_rsp_type_t sd_app_command(SDState *sd,
     case 22:  /* ACMD22: SEND_NUM_WR_BLOCKS */
         switch (sd->state) {
         case sd_transfer_state:
-            *(uint32_t *) sd->data = sd->blk_written;
-
+            stl_be_p(sd->data, sd->blk_written);
             sd->state = sd_sendingdata_state;
             sd->data_start = 0;
             sd->data_offset = 0;
-- 
2.41.0