[netdevice] Clear network device setting before unregistering

Avoid memory leaks by clearing any (non-child) settings immediately
before unregistering the network device settings block.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/netdevice.c

  err_probe:
 	for_each_table_entry_continue_reverse ( driver, NET_DRIVERS )
 		driver->remove ( netdev );
+	clear_settings ( netdev_settings ( netdev ) );
 	unregister_settings ( netdev_settings ( netdev ) );
  err_register_settings:
 	return rc;
 		driver->remove ( netdev );
 
 	/* Unregister per-netdev configuration settings */
+	clear_settings ( netdev_settings ( netdev ) );
 	unregister_settings ( netdev_settings ( netdev ) );
 
 	/* Remove from device list */