Documentation usb serial: fixed how to provide vendor and product id

Documentation usb serial: fixed how to provide vendor and product id

While trying to test a Cinterion GSM/GPS/3G module I had reconfigured
the USB interface by mistake and therefore needed to run a different
USB driver than CDC-ACM. It turned out that I need the "usbserial" driver.

This file is an official description how to use it:
Documentation/usb/usb-serial.txt

But it is outdated. The parameters vendor= and product= have been
superseded by a /sys interface.

Here was the solution:

https://bbs.archlinux.org/viewtopic.php?id=175499

  insmod usbserial vendor=0x#### product=0x####

becomes (first #### is vendor, second is product)

  modprobe usbserial
  echo #### #### >/sys/bus/usb-serial/drivers/generic/new_id

This patch changes the documentation file to describe the modern variant.
Please note that the old one still works (if compiled as module).

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Marek Belisko <marek@goldelico.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>