usb: serial: fix memory leak in generic driver

Fix a regression introduced by commit
715b1dc01fe44537e8fce9566e4bb48d6821d84b ("USB: usb_debug,
usb_generic_serial: implement multi urb write").

URB transfer buffer was never freed when using multi-urb writes.
Currently the only driver enabling multi-urb writes is usb_debug.

Signed-off-by: Johan Hovold <[e-mail]>
Cc: Greg KH <[e-mail]>
Acked-by: Jason Wessel <[e-mail]>
Cc: stable <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/serial/generic.c

···
 	dbg("%s - port %d", __func__, port->number);
 
 	if (port->serial->type->max_in_flight_urbs) {
+		kfree(urb->transfer_buffer);
+
 		spin_lock_irqsave(&port->lock, flags);
 		--port->urbs_in_flight;
 		port->tx_bytes_flight -= urb->transfer_buffer_length;