Supporting higher baud rates in newer FTDI chips

[Ian Lepore]

I have added support to the uftdi driver for the higher baud rates
supported by the latest H-series chips.  I hope to commit the changes
soon; the patch is attached for review.

In the process I discovered we've been setting the rate wrong on the
older chips (we put the high bit of the fractional rate in the wrong
place, but it only affects very high non-standard bit rates).  Because
the old code was wrong I ended up basically rewriting everything related
to calculating and setting the baud rate.  

The newer chips use the same product ID as the earlier ones, so I wrote
some new code to use the bcdDevice field from the device descriptor to
determine the model and version of chip.

I've tested this on all the chips I have around: 232R, 2232C, 2232D,
2232H, and 4232H.

-- Ian

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