sandbox: fix fall through in sandbox_flash_bulk()

Handling of SANDBOX_FLASH_EP_OUT should never fall through to
SANDBOX_FLASH_EP_IN.

This addresses a warning shown when compiling with
-Wimplicit-fallthrough.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/usb/emul/sandbox_flash.c b/drivers/usb/emul/sandbox_flash.c
index 01ccc4b..7c5c1ab 100644
--- a/drivers/usb/emul/sandbox_flash.c
+++ b/drivers/usb/emul/sandbox_flash.c
@@ -266,6 +266,7 @@
 		default:
 			break;
 		}
+		break;
 	case SANDBOX_FLASH_EP_IN:
 		switch (info->phase) {
 		case SCSIPH_DATA: