usb: pl2303: also use the divisor based baud rate encoding method for baud rates < 115200 with HX chips

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>