USB: serial: ftdi_sio: fix 300 bps rate for SIO

commit 7bd7ad3c upstream.

The 300 bps rate of SIO devices has been mapped to 9600 bps since
2003... Let's fix the regression.

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