USB: serial: cp210x: generalise CP2102N line-speed handling

The CP2102N equations for determining the actual baud rate can be used
also for other device types, so let's factor it out.

Note that this removes the now unused cp210x_is_cp2102n() helper.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>