I don't know how Net- or Open- handle this, but
FreeBSD returns the number of transmited bytes in
req.ucr_actlen, not in req.ucr_request.wLength.
(/usr/include/dev/usb/usb.h:544:
        int     ucr_actlen;             /* actual length transferred */

So then:
--- bsd.c       Sat Mar  4 03:52:46 2006
+++ ../../bsd.c Thu Aug  3 19:19:15 2006
@@ -476,8 +476,11 @@
   if (ret < 0)
     USB_ERROR_STR(-errno, "error sending control message: %s",
                   strerror(errno));
-
+#ifdef __FreeBSD_kernel__
+  return req.ucr_actlen;
+#else
   return UGETW(req.ucr_request.wLength);
+#endif
 }
 
 int usb_os_find_busses(struct usb_bus **busses)