USB: serial: allow custom multi-urb write bulk callbacks

Allow drivers to implement their own multi-urb write bulk callbacks as
we do for single urb writes.
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/generic.c

@@ -217,7 +217,7 @@ static int usb_serial_multi_urb_write(struct tty_struct *tty,
 			usb_sndbulkpipe(port->serial->dev,
 					port->bulk_out_endpointAddress),
 			buffer, towrite,
-			usb_serial_generic_write_bulk_callback, port);
+			port->serial->type->write_bulk_callback, port);
 
 		status = usb_submit_urb(urb, GFP_ATOMIC);
 		if (status) {