The IRQ values are defined few lines earlier, use them instead of
the magic numbers.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20201017180731.1165871-3-f4bug@amsat.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>