USB: cp210x: clean up, refactor and document speed handling

Clean up and refactor speed handling.
Document baud rate handling for CP210{1,2,4,5,10}.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Cc: Preston Fick <preston.fick@silabs.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>