usb: pl2303: also use the divisor based baud rate encoding method for baud rates...

Now that the divisor based baud rate encoding method has been fixed and
extended, it can also be used for baud rates < 115200 baud with HX
chips.
This makes it possible to adjust the baud rate almost continuously
instead of just beeing able to select between 16 fixed standard values.

Tested with a PL2303HX 04463A (week 46, 2004, rev 3A).

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