Rename usbserial to usbserial_ftdi

bus/usb/usb.c

@@ -270,7 +270,8 @@ void grub_usb_device_attach (grub_usb_device_t dev)
 	  grub_dl_load ("usb_keyboard");
 	  break;
 	case 0xff:
-	  grub_dl_load ("usbserial");
+	  /* FIXME: don't load useless modules.  */
+	  grub_dl_load ("usbserial_ftdi");
 	  break;
 	}
     }