USB: serial: pl2303: fix non-supported xon/xoff

Older pl2303 devices do not support automatic xon/xoff flow control, so
add add a flag to prevent trying to enable it for legacy device types.

Refactor the IXON test into a helper function to improve readability.

Fixes: 7041d9c3 ("USB: serial: pl2303: add support for tx xon/xoff flow control")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>