USB: pl2303: use divisors for unsupported baud rates

Use direct method for supported baud rates, otherwise use divisors.
Limit baud rate to 12 Mbaud with HX type.

This change has been tested to work with PL-2303HX at 115200, 500000,
1000000, 2000000, 2500000, 3000000 and 4000000 baud rates.

Signed-off-by: Lauri Hintsala <lauri.hintsala@bluegiga.com>
Signed-off-by: Johan Hovold <johan@kernel.org>