Add minimum baud rate to the cp210x driver.

According to the datasheet for CP2105, the SCI supports 2400 as the
lowest baud rate. As this is not heeded in the current code, an error
message 'failed set req 0x1e size 4 status: -32' when trying to set a
lower baud rate such as 300.
The other cp210x models to date supports a minimum baud rate of 300.
Signed-off-by: Johanna Abrahamsson <johanna.abrahamsson@afconsult.com>
[ johan: simplify min_speed init, move clamp after comment, and drop
         unused serial-data pointer from cp210x_get_actual_rate() ]
Signed-off-by: Johan Hovold <johan@kernel.org>