superio: Don't use MAX_SERIAL_PORTS for serial port limit

superio: Don't use MAX_SERIAL_PORTS for serial port limit

The superio device has a limit on the number of serial
ports it supports which is really only there because
it has a fixed-size array serial[]. This limit isn't
related particularly to the global MAX_SERIAL_PORTS limit,
so use a different #define for it.

(In practice the users of superio only ever want 2 serial ports.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180420145249.32435-12-peter.maydell@linaro.org