we don't implement it, but can see firmwares using it via UNIMP logs.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
 hw/sd/sd.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/hw/sd/sd.c b/hw/sd/sd.c
index 7e9d7366fe..2b4254802f 100644
--- a/hw/sd/sd.c
+++ b/hw/sd/sd.c
@@ -1438,6 +1438,9 @@ static sd_rsp_type_t sd_app_command(SDState *sd,
         }
         break;
 
+    case 55:    /* Not exist */
+        break;
+
     default:
         /* Fall back to standard commands.  */
         return sd_normal_command(sd, req);
-- 
2.15.1