usb serial: use new descriptor infrastructure.

Switch the usb serial drivers (serial, braille) over to the
new descriptor infrastructure.

Note that this removes the freely configurable vendor and product id
properties.  I think the only reason this was configurable is that the
only difference between the serial and the braille device is the
vendor+product id.  Of course the serial and braille devices keep their
different IDs, but they can't be overritten from the command line any
more.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>