usb: pl2303: add two comments concerning the supported baud rates with HX chips

I've found some new datasheets which describe some additionally
supported standard baud rates and I've verified them with my HX
(rev. 3A) device. But adding support for individual (chip type
specific) baud rates would add a good amount of extra code (especially
when support for further chips will be added to the driver one day),
which makes no sense as long as we are not using the direct baud rate
encoding method for newer chips.
So for now, just drop a comment about these additionally supported baud
rates.

The second comment is about the baud rate differences between the two
encoding methods. In theory, we could optimize the code a bit by
comparing the resulting baud rates of both methods and selecting the
one which is closer to the requested baud rate. But that seems to be a
bit overkill, because the differences are very small and the device
likely uses the same baud rate generator for both methods so that the
resulting baud rate would be the same.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>