USB: serial: ftdi_sio: Add support for ICP DAS I-756xU devices

commit ea6db90e750328068837bed34cb1302b7a177339 upstream.

A Fedora user reports that the ftdi_sio driver works properly for the
ICP DAS I-7561U device.  Further, the user manual for these devices
instructs users to load the driver and add the ids using the sysfs
interface.

Add support for these in the driver directly so that the devices work
out of the box instead of needing manual configuration.

Reported-by: <thesource@mail.ru>
Signed-off-by: Josh Boyer <jwboyer@fedoraproject.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>