USB: add SeaLevel 2106 SeaLINK support to ftdi_sio

We have a couple of these USB-Serial converters around; they're slightly
different from the 2104 models in that they can handle 500Kb/sec over RS422.
The existing ftdi driver seems to work just fine if we add in the
appropriate IDs.

Patch is against 2.6.17.6, but should apply cleanly to pretty much
anything recent.

From: Justin Carlson <justinca@qatar.cmu.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>