usbhost: Make unplugging hubs more reliable.

USB drivers should not try control devices in the usbhost_disconnected
function. The control endpoint is NULL at this point because the device
has already been disconnected.

Before this change, the hub driver crashed when the child called
usbhost_hubpwr after the parent had already deactivated the control
pipe and set hport->ep0 to NULL.

drivers/usbhost/usbhost_hub.c

@@ -1095,10 +1095,6 @@ static void usbhost_disconnect_event(FAR void *arg)
 
   work_cancel(LPWORK, &priv->work);
 
-  /* Disable power to all downstream ports */
-
-  usbhost_hubpwr(priv, hport, false);
-
   /* Free the allocated control request */
 
   DRVR_FREE(hport->drvr, (FAR uint8_t *)priv->ctrlreq);