hw/isa/piix4: Correct IRQRC[A:D] reset values

hw/isa/piix4: Correct IRQRC[A:D] reset values

IRQRC[A:D] registers reset value is 0x80. We were forcing
the MIPS Malta machine routing to be able to boot a Linux
kernel without any bootloader.
We now have these registers initialized in the Malta machine
write_bootloader(), so we can use the correct reset values.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20221027204720.33611-4-philmd@linaro.org>