usbus_fmt: assert descr len matches claimed len

This commit adds runtime assertions to validate that the total length of
the configuration descriptor as communicated to the host device matches the
generated length of the configuration descriptor.

sys/usb/usbus/usbus_fmt.c

@@ -260,6 +260,7 @@ size_t usbus_fmt_descriptor_conf(usbus_t *usbus)
     usbus_control_slicer_put_bytes(usbus, (uint8_t *)&conf, sizeof(conf));
     len += _fmt_descriptors_post(usbus, usbus->descr_gen);
     len += _fmt_descriptors_ifaces(usbus);
+    assert(len == conf.total_length);
     return len;
 }