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>

diff --git a/drivers/usb/serial/generic.c b/drivers/usb/serial/generic.c
index 9756b5c22..be52c74 100644
--- a/drivers/usb/serial/generic.c
+++ b/drivers/usb/serial/generic.c
@@ -217,7 +217,7 @@
 			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) {