USB: serial: ftdi_sio: fix IXON/IXOFF mixup

Since forever this driver has had IXON and IXOFF mixed up, and has used
the latter rather than the former to enable hardware-assisted software
flow control on output.

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