USB: cp210x: do not map baud rates to B0

commit be125d9c upstream.

We do not implement B0 hangup yet so map low baudrates to 300bps.

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